Selasa, 21 Juli 2015
An Introduction to Gemology
by Don Clark CSM
Gemology
is the study of gemstones. Some dictionaries define it as the
“scientific study of gemstones; it is almost impossible to remove the
scientific element. There may be investors whose only interest is in the
value of the stones, but if they ever need to distinguish one gem from
another, they are dealing with science.There are many categories of gemologists. Jewelers need to understand gemology in order to be able to answer their customer’s’ questions and identify the gems brought to them.
The goldsmith needs specific knowledge about the physical characteristics of gems. A setting that would be ideal for a diamond would be inappropriate for an opal and vice versa. The amount of pressure used to set the prongs on a garnet would break a tanzanite.
Some gems will withstand the heat of repair work that involves high temperature soldering. Some can be left in the setting if steps are taken to moderate the amount of heat they receive. Still others are so heat sensitive they need to be removed.
The lapidary also needs special knowledge. Cutting and polishing techniques vary from gem to gem. What would work well for one material would be a waste of time on another and disastrous on something else.
When faceting, thought needs to be given to color management. How the rough is oriented can make a lot of difference in the appearance of the finished gem. The style of cutting is also a part of color management. The choice of cut can lighten or darken a gem, which will considerably affect both the appearance and the value of the stone.
The choice of a cut, which includes the shape and number and location of facets, also influences the brilliance of the gem. The angles at which the facets are cut must be carefully chosen. These factors are balanced, or compromises made, so as to not sacrifice too much material in the pursuit of beauty.
Another category of gemologist is the scientist. These are people with degrees in geology, chemistry and sometimes physics. While this is one of the smallest categories of gemologists, they are at the same time one of the most influential.
At the heart of gemology is gem identification. Some rubies and garnets are impossible to tell apart from each other by observation, but their values are considerably different. A precise and accurate means to distinguish them is absolutely necessary.
When dealing with whole crystals, the ruby and garnet are easy to distinguish. Garnets form in the cubic system. While they vary in shape, they tend to be roundish and the number of sides is always a multiple of four. Rubies, on the other hand, form long thin crystals. They are in the hexagonal system and always have six sides.
Most of the raw material that is cut into a finished gem isn’’t found in whole crystals, but in broken pieces. Using the techniques of mineralogy, they are easily distinguished from each other. Scratch tests, where the unknown gem is scratched by various substances, will determine its hardness. Other useful tests are the reaction to acids and the flame of a blow torch. These are categorized as destructive tests and are obviously inappropriate for cut gems.
For centuries it was the lapidary who was in a position to most easily recognize the differences in similar-appearing gems. During the cutting process, gems get viewed intently, a perspective that no other gemologist has. Identifying inclusions are given a lot of attention, then as many as possible removed. Differences in hardness are readily apparent when cutting and polishing, as are other characteristics.
A method needed to be devised where cut gems could be identified without damage. To this end scientists began to first identify the measurable physical and optical properties of gems. Next they devised instruments to measure these properties. There was a long process of systematically measuring and recording these properties so they could be researched. (Though well established, this is actually an ongoing process.) Eventually all this was put together into methods that could be used by people without extensive scientific backgrounds or large and expensive laboratory equipment.
That is not to say that it doesn’’t require substantial education to identify gems. It is a large and complex subject that is continuing to increase in complexity as new gems are discovered and new ones are created in the laboratory. However, one doesn’’t need a degree in chemistry or physics to simply measure the properties of our gems. The most esoteric part was discovering those properties and creating the tools to measure them.
If you are interested in learning about gems the first step would be to learn how they are categorized. Also important in the early stages is learning the terminology used to describe gems. Next you can learn what the physical and optical properties are. When you have this background, you can get into gem identification.
Of course there are many side roads to travel. You may find a fascination with phenomenal gems or their inclusions. Many people find a desire to collect gems and this often leads to making jewelry or learning how to cut gems.
Whether your interest is casual or professional, there is much to delight and amaze. It is something you can do from your desk, or something that allows you to get your hands dirty. Plus, the subject of gemology is one of those where you will never run out of new elements to discover!
Specific Gravity Testing
(
This is Premium Content. You are a Gold Member and therefore are viewing this entire article.)
This is Premium Content. You are a Gold Member and therefore are viewing this entire article.)
Table of Contents [hide]
Specific gravity (SG) is also known as relative density or just density. Technically, density is mass per unit of volume or— the ratio of the amount of matter in an object compared to its volume. You know that if you had a piece of plastic and a piece of lead the same size, the lead would be much heavier. That is density, how much something weighs in relationship to its size.
Specific gravity is a case of relative density, which is defined as the ratio of the density of one substance to that of another — in our case a gemstone to water. To put it another way, specific gravity is the relationship of the weight of a gemstone to an equivalent volume of water. If a gem has a specific gravity of 3, that means that it weighs three times as much as the same amount of water.
The distinction between these terms is important in some sciences, but in gemology we use the terms density and specific gravity interchangeably. It is acceptable to say that a gem has either the specific gravity or density of 3. In either case we mean that it weighs three times as much as the same amount of water.
While you can determine how much volume a gem has, and then weigh that amount of water, you do no’t have to approach it in this manner. There is a simple method for determining the SG by first weighing it in air, then again under water using this formula:
weight of the gem in air / (weight of the gem in air – the weight of the gem under water.)
For example, you measure a gem normally and get a weight of 1.6 carats. Next you measure it under water and come up with .7 carats. Take your first reading, 1.6, and subtract .7 to come up with .9 for the divisor. Now, divide 1.6 by .9 and you have a specific gravity of 1.77.
Testing Methods
There are two methods commonly used by gemologists to determine specific gravity. One is the use of heavy liquids and the other is with a hydrostatic scale. Both methods have their advocates.
Some feel that heavy liquids are faster and are more accurate on small stones; however, there are multiple disadvantages associated with this method. Many of these heavy liquids are toxic and you are advised not to inhale the fumes or get any on your skin. They are also flammable, requiring careful storage and handling. Finally, you can only get an estimate with this method. While an estimate is sometimes good enough, there are other times when you have a borderline reading, making it impossible to make a separation.
Measuring specific gravity with a scale is somewhat more time consuming, but usually more accurate. It also involves less hazards. For these reasons, the IGS recommends using a hydrostatic scale.
Measuring SG with a Balance Scale
Almost any balance-type scale can be adapted for specific gravity testing. All you need to do is to find a way to submerge one pan in water. If you look in your cupboard you will certainly find something that will suit this purpose.
Take an appropriate sized cup or jar and place it so the weighing pan will rest comfortably inside it. It must ride freely without touching any of the sides.
Weigh the gem as usual. Then, fill the cup or jar with enough water that your gem will be completely submerged. Place your gem on the pan that is under water. Take care that no air bubbles are on the surface of your gem, then weigh it again.
Making a SG Scale
If you have an electronic scale, you can make a simple balance beam to go along with it that will serve to measure SG. The key factor to consider when making a balance beam is to get it as sensitive as possible. The sensitivity is determined by two elements, the width of the beam and how narrow the fulcrum is. For example, a two foot long beam will be more sensitive to changes of weight on the ends, than one that is just three or four inches long. It will also be more sensitive if it is balance on a sharp edge, rather than a rounded or dull one.
You are going to need pans hanging from each end for weighing. They can be of almost any material you have handy: bottle caps, aluminum foil, etc. In the center, or off one end, you will need to put a needle. Make a mark on a stationary part of the scale to match the needle with when the scale is in balance. Then find a vessel that one of the pans can rest freely in so you can conduct your submerged measurements.
If you make such a balance beam, you do no’t need an expensive set of calibrated weights. Instead, just get your scale in balance by adding enough sand, salt, or other substance. When you have your scale balanced, weigh the salt or sand on your electronic scale.
The Hanneman SG Scale
Hanneman’s Gemological Instruments has a special scale for determining density that does no’t require any math. Instead it has a printed scale on one side of the balance beam so you can read the specific gravity directly. It still requires you to weigh the gem twice, once in air, then again under water. However, for the second reading you move the counter weight towards the fulcrum. When the scale is in balance, you read the SG where the counter weight lies over the printed scale.
Hint: This can be an awkward piece of equipment to use at first. To get it balanced, you need to put a weight on one of the arms. This weight can move during measurements, so always double check that the scale is centered after each reading. If you glue your weight to the beam you can avoid this problem. The glue will change the balance of your beam, so take that into consideration and be prepared to balance the scale before it dries!
Measuring Specific Gravity with an Electronic Scale
If you have an electronic scale, you can design a simple accessory for your scale that will allow you to gather the data needed to calculate specific gravity. It should be made from very light wire. I used a piece of 26-gauge galvanized steel wire in constructing my specific gravity accessory. This wire is used for flower arrangements and is available in most craft stores. A 12-inch long piece weighs slightly over two carats, allowing you the full range of your scale for weighing.
Begin by bending a circular base that is a bit smaller than the weighing pan on your scale. Bend the long end so it rises directly up from the base about three inches, and then give it a bend so the end passes over the center of the base. Now bend it down, directly over the center.
The next step is to create a spiral basket to hold the stone. It is important that the basket is centered over the base. If it is not centered, the apparatus will be wobbly and fall over easily. Use plenty of wire for the basket so it can be enlarged or reduced for different size gems.
Finish by making a visible mark, just above the basket. This step is critical, as you will see in the instructions.
This simply specific gravity accessory allows you to weigh gems with an electronic scale.
You also need a water container that will slide easily around the basket and be deep enough to cover a good sized stone with room to spare. This can be a beaker, a bottle cap, shot glass, or an egg cup. What is important is that you can completely submerge the stone without touching the gem or apparatus. When you use the water container, do not fill it completely. One of your priorities is to not spill any water on your scale!
The weight of the wire is going to change when you submerge it. To find out how much it changes, put your water container over the basket and raise the water level to the mark you made on the wire. Write down the scale reading. Do this a few times for practice. If you always raise the water to the same level, your readings will be consistent.
To use the accessory, first weight the stone dry and record the weight. Next, set the SG accessory on the scale and press the tare button. That will set the scale back to zero.
Put the stone in the basket and note the weight. It should be the same as before. If not, zero the scale and try again. Lift the water container around the gem so the water level is up to the mark on the wire. The gem should be completely submerged. Look at the weight as you do this. If the water container is touching the wire, the weight will bounce up and down. When the stone is submerged and not touching the container, take your reading.
You now have three readings, the weight of the stone dry, the weight of the stone under water, and the weight of the wire basket under water. The latter will have a reading somewhere around .06 carats. Add that to the submerged weight of the stone. This is your actual weight of the stone under water. For example, if the stone weighed 1.39 carats, add .06, for a total of 1.45.
The final step is the math. Here is the formula for determining specific gravity:
weight of the gem in air / (weight of the gem in air – the weight of the gem under water.)
Refining Your Techniques by Raul Berenguel, PhD Invited Researcher at CTEC, (Fernando Pessoa University, Porto)
Getting accurate readings is critical to your success. If you make an error of .01 carats weighing a ten carat gem, it will only affect the results by .03. That is not significant. On a one carat stone, a .01 weight error will make a .08 difference in your specific gravity calculations. That is enough to misidentify some stones. On a half carat stone the same error changes the specific gravity reading by .13. With smaller gems the results are amplified even more.
Here are the detailed procedures to ensure maximum accuracy:
1) You can use any liquid for specific gravity testing. Water is the most common, but you should never use tap water. Its density is not constant and is always higher than distilled water. The table below refers to pure, distilled water, which you can find in any supermarket.
For the the most accurate results use toluene. Its surface tension is much lower than water. Toluene is recommended by B. W. Anderson, BSC, FGA, FKC, Founder and First Director of the Gems Identification Laboratory of the Chamber of Commerce of London in his book “Gem Testing – 9th Edition.
NOTE: Toluene is a solvent and should not be used with delicate or porous gems, such as pearls and turquoise. It should also be avoided for stones with fracture fillings or glued doublets.
2) Take in account the following factors regarding your scale. First, what is the minimum reading of your balance? If it is, let’s say, 0.20 ct, you will not have good results measuring values near that.
Another important issue is the accuracy of your balance. Look in your scale’s manual for this value. As an example, you could have a cheap balance that weighs up to 200 carats with a precision of 0.01 carats, but the accuracy is +/- 0.02. If you read 31.03 carats, that means a value between 31.01 carats and 31.05 carats.
3) Always calibrate your balance if you move it a long way or if the temperature has changed significantly since the last calibration.
4) Remove any air bubbles in the spiral or pan that will receive the gem with a needle or other clean instrument.
5) The gem must be perfectly clean. Do not handle the stone with your fingers after cleaning, use tweezers instead.
6) You can use the carat mode or the gram mode. However, if you set your balance to the gram mode, you will work with more decimals.
7) Weigh the stone at least five times. Disregard any measurement that varies from the rest of the readings. For instance, you got 4.567, 4.580, 4.568, 4.556 and 4.601. This last one should not be taken into account and you should do another reading to replace it. From this, you should take the mean value, or average, and round the value for the same number of decimals you use in the readings, in this case 3. Now you have the weight of the gem in the air — let’s call it Wa.
8) If you are using a balance beam with a stationary cup it is very important to zero the balance with the pan immersed in the liquid.
9) Now weigh the gem immersed in the liquid. Use the same method as in step 7, but zero the balance before every measurement, because some liquid will be lost each time you remove the gem for another reading. Now you have the gem weight in the liquid — let’s call this Wl.
10) Subtract Wl from Wa. Let’s called this Wp. This is the gem loss of weight in the liquid.
11) Now divide Wa by Wp and you have the approximate density of the gem.
12) To be accurate, you must multiply this last value by the correction factor for the temperature of the liquid. Assuming that the bottle is in the lab more than half an hour, you can use the atmospheric temperature. This factor is simply the density of the selected liquid you chose at that temperature.
13) Use this table as a correction factor;
| Temperature Celsius | Temperature Fahrenheit | Toluene | Distilled Water |
|---|
| 10 | 50 | 0.876 | 0.9997 |
| 11 | 51.8 | 0.875 | 0.9996 |
| 12 | 53.6 | 0.874 | 0.9995 |
| 13 | 55.4 | 0.873 | 0.9994 |
| 14 | 57.2 | 0.872 | 0.9993 |
| 15 | 59 | 0.871 | 0.9991 |
| 16 | 60.8 | 0.870 | 0.9990 |
| 17 | 62.6 | 0.870 | 0.9988 |
| 18 | 64.4 | 0.869 | 0.9986 |
| 19 | 66.2 | 0.868 | 0.9984 |
| 20 | 68 | 0.867 | 0.9982 |
| 21 | 69.8 | 0.866 | 0.9980 |
| 22 | 71.6 | 0.865 | 0.9978 |
| 23 | 73.4 | 0.864 | 0.9976 |
| 24 | 75.2 | 9.863 | 0.9973 |
| 25 | 77 | 0.862 | 0.9971 |
| 26 | 78.8 | 0.861 | 0.9968 |
| 27 | 80.6 | 0.861 | 0.9965 |
| 28 | 82.4 | 0.869 | 0.9963 |
| 29 | 84.2 | 0.859 | 0.9960 |
| 30 | 86 | 0.858 | 0.9957 |
If you choose to use distilled water, you can minimize the surface tension by adding a bit of liquid soap or detergent. Two or three drops in a quart, litre, or more will not make a significant difference in the density of the water.
Finally, realize that pure minerals will always have the same density, but we rarely encounter pure materials in the gem world. They have impurities, bubbles and inclusions of other minerals that effect their density. The range of densities listed in reference works reflects what you are most likely to find.
You need to keep these variations in mind when testing. There are occasions when inclusions will take your gem out of the normal range. An obvious example is a piece of amber with several large air bubbles. They will have a significant effect on the specific gravity of the piece. Stones with lots of dense hematite are also likely to be outside the normal range for that species.
Testing with Heavy Liquids
“ Heavy Liquids” are liquids that are chosen for their density. Many are formulated by mixing ingredients to come up with the most useful properties. They are mostly solvents, in the same family as paint thinners. Care needs to be taken not to inhale the vapors or get any on your skin or clothing. Also be careful of heat sources, as these are flammable.
The values of heavy liquids need to be monitored. They can be changed by adding stones that haven’t been thoroughly cleaned. If they are a mixture of liquids, the individual ingredients will have different evaporation rates. To maintain their accuracy, the liquids need to be calibrated from time to time. Read the manufacturer’s instructions on how to do this.
To be the most useful, heavy liquids come in sets. The GIA set has densities of 2.57, 2.62, 2.67, 3.05, and 3.32 Other manufacturers choose different ranges, but they are similar.
The theory behind using heavy liquids is simple. If you put a gem in a liquid with the same density, it will stay in place. If it has a greater density it will sink and it will float if it has a lower density. The speed at which the gem sinks or floats will give you an idea of how much the SG varies from the liquid. If the difference is just a couple hundredth of a point, it will drift slowly to the top or bottom. If it is a couple of tenths, it will move faster. If it is a full point or more, it will sink like a rock or pop to the surface like a cork.
When using the liquids, you have to take the shape of the gem into consideration. A thin, flat cabochon will move at a different rate if it is placed in the liquid flat side down, than if it is on edge. It could move faster or slower than a faceted gem, so you need to keep this in mind as you do your testing.
The actual testing procedure is a matter of elimination. Take a guess at which liquid has a density closest to your gem and, with a pair of tweezers, put the gem in the middle of the bottle. Note if it sinks or floats and how fast. Take the gem out and clean the liquid off of it. Then place it in the next liquid, higher or lower in SG value, depending on the results of your initial test. Keep doing this until you find the liquid that is closest to the density of your gem.
Your final step is to make an estimation based on your observations. For example, say your stone floated slowly in the 2.67 and sank a bit faster in the 3.05 liquid. The median measurement is 2.86, but it floated a bit faster than it sunk. That would indicate the gem has a specific gravity slightly lower than 2.86, around 2.80 to 2.82.
Special Liquid for Testing Amber
Amber and its imitations have much lower SG’s than any of the heavy liquids. A handy testing liquid can be made by boiling water and adding as much salt as you can dissolve in it. This will have a density of about 1.13. Amber, with a SG of 1.10 will float in this solution. Most of its imitations will sink.
A few plastics have a specific gravity as low as 1.05 and many can be lower than amber if they have significant air bubbles inside. So, if your sample sinks, you can be sure it is not amber. If it floats you need to determine if it is plastic or amber. The RI will distinguish plastic if it varies from amber. However, since amber and plastic can both have an RI of 1.54, it will no’t tell you for certain if it is amber. Since they also share so many visual characteristics, you will probably have to use a hot point to distinguish them.
This is considered a destructive test, but with care it can be done almost invisibly. Be sure to practice on your own material and ask permission before doing a hot point test on a customer’s gem. Find a place on your gem where a mark would be as unobtrusive as possible. This is usually on the bottom, an edge, or an area with existing scratches. Next, heat the tip of a needle until it glows red. Touch the selected spot just enough to release a tiny whiff of smoke.
Now for the hard part, smell the smoke. If it is amber, the smell is of fine incense. If it is chemical and offensive, it is plastic. This is another reason to make your test on as small a scale as possible!
Ultraviolet Testing
( This is Premium Content. You are a Gold Member and therefore are viewing this entire article.)
This is Premium Content. You are a Gold Member and therefore are viewing this entire article.)
By Donald Clark CSM
Identifying
an unknown gem begins with measuring the refractive index (RI) and
specific gravity (SG). These two tests will leave you with a short list
of possible materials; however, further testing is necessary to reach a
positive identification. Checking a gem’’s reaction to ultraviolet light
is one of the easiest tests you can perform. Often ultraviolet testing
is the only other test you need to perform to make a positive
identification.The oils used as fillers in emeralds and other gems often fluoresce. The fluorescence of the enhanced layer of a diffusion treated gem is usually different from the rest of the stone. Components of assembled stones usually have different fluorescent qualities as well. Seeing these in an early stage of your examination can save time later.
Tools
| Caution!Short wave ultraviolet can damage your eyes, even to the point of blindness! Never look at your UV light. If you are unsure if it is on, check the switch. Better yet, a flame-fusion ruby will tell you instantly if the light is on or not. |
For gem testing purposes, you need lights that will show the long and short waves independent of each other. You can buy these as separate units, or a single unit that switches from one to the other.
UV lamps range from four to one hundred watts. The stronger the light, the easier it is to see the fluorescence; however, the price goes up along with the wattage. Even the smallest, battery-operated light will do for standard testing, provided you are careful to eliminate all extraneous light. The only caveat is transparency testing. Only a high-powered light will work for this purpose.
Besides the light, you need a viewing cabinet. There are many good ones available that are matched to particular lights. You can also make a viewing cabinet, if you are on a budget or do not like any of those that are compatible with your UV light. First, find a cardboard box that is slightly larger than your lamp. Cut a hole in the top for the light. Then, cut an opening in the front for viewing. Finish the box by painting it flat black.
This diagram shows the setup of a simple viewing box for your UV light.
UV fluorescence is faint, so you need to place the test stones close to the light source. If you have a powerful light, four or five inches is about right. If you have a lower wattage light, you will want the stones as close as one inch.
Commercial viewing cabinets come with a contrast control filter. This helps to filter out extraneous light. You can buy contrast control spectacles that are made of the same material, but if your viewing cabinet is tight enough, you will not need one.
Judging fluorescence is easier if you have a comparison stone. Cheap, flame-fusion ruby is ideal for this purpose. (It will also tell you if your light is on.) It fluoresces strongly to both short wave and long wave UV. The only caution is to not put it too close to your test stones because it fluoresces so strongly that it can mask a light reaction.
Testing Procedures
Before you start, realize that fluorescent colors are completely unrelated to the body color of a gem. In most cases, they will be entirely different. For example, green emerald fluoresces red. Also, realize that you can get entirely different results from long wave and short wave UV. There are no shortcuts, every step is important.
Clean the stone carefully before testing. Lint and oils have their own fluorescence that will confuse what you see. Place the test stone in the viewing cabinet next to your comparison stone. Dim the room by closing the curtains and turning off all other lights. Turn the UV light to short wave. If you have contrast control spectacles, put them on. Now wait a bit for your eyes to adjust to the lower light.
Depending on your viewing cabinet, you may have to adjust the distance between the lamp and the gem. Move it up and down until you can see the fluorescence clearly. In most cases, you will move it closer, but if the fluorescence is strong, you may want to move it away from the gem.
Move the gem so you can view it from different directions. Use tweezers that do not fluoresce to maneuver your stone. You can tape the ends of your tweezers if this is a problem. If the reaction is weak, move your test gem away from the comparison stone; its high fluorescence can make it hard to see a weak reaction.
The top of this doublet is inert. Also note dust specks on pavilion.
When assessing fluorescence, you are looking for three things:
1. The hue
2. The intensity of fluorescence — it can be weak, moderate, strong, or inert.
3. If the color is evenly distributed, zoned, or patchy
If you have an uneven distribution of color, it can be a result of inclusions, which sometimes helps with identification. Assembled stones are readily apparent by their reaction. The crown and pavilion will be distinctively different, if they are made from different materials. Oils and fillers frequently fluoresce differently than the host material. They are recognized by their shape and location in the gem.
Turn your UV light off and observe if there is any phosphorescence, which is a glow that remains after the lights are extinguished. This is useful in some identifications, as few gems phosphoresce. An important example is natural opal, which phosphoresces longer than its synthetic imitations. In this case, you need synthetic opal comparison stones to make the distinction.
Turn on enough room lights to write comfortably and record what you observed. Then repeat the procedure with long wave ultraviolet.
UV Transparency
Some gems are transparent to ultraviolet light. Most notably, Type IIa diamonds, the ones used for HPHT treatment, are transparent to short wave ultraviolet light. There are two methods of testing for UV transparency:
Basic UV Transparency Test
Create a shield with a hole for the gem. Place a piece of highly fluorescent material under the hole. Scheelite is frequently recommended, but it only reacts to short wave. You can use a slice of synthetic ruby or any other highly fluorescent material.
Before trying this test, check your mineral to see how it reacts to both long and short wave UV. If it only fluoresces under one, than that the only thing you can test for.
Orient the gem so it rests between the light source and the reaction mineral. Just a small amount of light will pass through and the reaction will be small as well. To aid seeing it, turn off all other lights.
Interpreting the results is simple. If your reaction mineral fluoresces, the test stone is transparent to UV. If it remains dark, the gem is opaque to UV.
This diagram shows the setup for a basic UV transparency test.
A similar test can be done using photographic materials. This requires total darkness, plus the ability to develop the paper.
Begin by making a shield, similar to the one above. With this method, you can also make extra holes for comparison stones of know values. In addition, you can run tests for both long and short wave ultraviolet.
For best results, use slow, contact printing paper. Place it emulsion side up in a flat bottom dish and cover it with water. Make sure the dish does not fluoresce to UV. Turn the UV light on for a fraction of a second. That is because the UV lamp gives off visible light as well.
Develop the paper as usual. If you see a white spot on the paper, the stone is transparent to UV. If the stone is opaque, you will see a light rim around a dark spot.
Destructive Tests
( This is Premium Content. You are a Gold Member and therefore are viewing this entire article.)
This is Premium Content. You are a Gold Member and therefore are viewing this entire article.)
By Donald Clark CSM
Geologists
have a much easier time of identifying minerals than the gemologist
does. They scratch gems to see how hard they are, check their reaction
to acids, and put them in the flame of a torch.These destructive tests are fine for the geologist,but they are obviously not very useful for the gemologist because causing damage reduces the value of a gem. This is why we use complex optical and microscopic examinations to determine the identity of our gems, rather than relying on these informative, yet damaging, tests. There are occasions, however, when our standard testing procedures fall short of positively identifying a gem and we do have to resort to destructive tests. With care, they can be used to a minute extent without diminishing the value of the stone.
Scratch Testing
This is one of the most destructive tests. Never perform a scratch test on a finished stone. Even if done in an inconspicuous area, it can fracture or even shatter the gem.
Scratch testing can be used on rough, but caution is needed. Realize that the rough may have incipient fractures that cannot be seen, or have internal stress that will open up easily to pressure. For these reasons, the testing should only be performed on small protrusions. Whenever possible, saw a piece off of the main stone for testing.
This photo shows a portion of this stone that could be cut off for scratch testing.
You do not need any other points because minerals with the same hardness will not scratch each other. Anything you cannot scratch with a piece of corundum, which is a 9 in hardness, must be a diamond, (hardness 10) or a Moissanite (hardness 9 ¼.)
The majority of our gems are seven or harder and we rarely work with stones softer than apatite. Hence, you will not need testing points for the extremely soft minerals either.
Scratch Test Procedures
To begin testing, guess which pencil is the same hardness as your specimen. With a moderately firm hand, try to scratch the mineral. Material rubbed off the hardness point resembles a scratch, so wipe the stone before inspecting.
You will have one of two reactions:
1. The stone has a scratch: You now know that your point is harder than the mineral. Try the next softer pencil and proceed working down in hardness until you find one that will not scratch your stone.
2. The stone is not scratched: Try the next harder pencil and continue working up in hardness until you find one that will scratch the stone.
You will end with two figures, the one that did scratch your stone and the one that did not. For example, an 8 pencil might have scratched the mineral and the 7 did not. By definition, materials of the same hardness will not scratch each other. Therefore, we know that the mineral being tested is between 7 and 8 in hardness.
Streak Tests
This is a fun test if you have rough material to spare. It is not recommended for finished gems, as there is a good chance the gem will break. Even with rough material, choose pieces you would not mind being ruined.
As you know, most gems receive their coloring from selective absorption. See The Spectroscope for more information on selective absorption. Before testing these gems, you should confirm that they do not have any coloring material in them. You can test this by rubbing a piece against a ceramic tile. The bit of material that rubs off, the streak, will be colorless. It does not matter how dark a sapphire or garnet is, it will always leave a colorless streak.
Since the majority of our gems leave a colorless streak, it is easier to note which ones do not. The accompanying chart lists the ones we are most likely to encounter. The only things not listed here are exceptionally rare. Any common gem not on the list will have a white streak.
| Gemstone | Streak Colors |
|---|
| Azurite | light blue |
| Chrysocolla | greenish white |
| Crocoite | orange |
| Dioptase | green |
| Epidote | gray |
| Hematite | red-brown |
| Imitation Hematite | black to reddish black |
| Lapis | very light blue |
| Imitation Lapis | dark blue |
| Malachite | green |
| Marcasite | greenish black |
| Pyrite | greenish black |
| Sphalerite | brown or white |
The procedures for streak testing are simple. First, go to a building supply store and buy a ceramic tile. Some tiles have rippled backs. You need to find one that has large flat areas on the back. We use ceramic tiles because they are hard enough that the gem will rub off on them.
Rub the material firmly against the back of the tile; you cannot do this on the glazed or shiny side. Now look at the streak it left and see what color it is. That is all there is to it!
Hot Point
A hot point will tell you if a material is natural or a plastic imitation. It will also let you know if a stone is treated with wax or plastic.
You can purchase electrically heated hot points or you can make your own by inserting the blunt end of a needle into a cork. To use, simply heat the needle over a small flame until the tip turns a dull red.
To Test for Plastic
Heat the hot point until the tip glows dull red. Place the stone close to your face and touch the hot point to an inconspicuous spot, usually near the girdle. Hold it there for a second or less, and then smell the smoke.
Amber smells resinous, like incense. Coral and shells have a protein smell, like burning hair. Jet has an oily smell, like coal, tar, or asphalt. Most plastics have an acrid smell, but very few have a protein odor.
Stones that are coated with plastic will have much less odor than solid ones. Try this on something you know is solid plastic first. It just takes a little practice to make this distinction.
Testing for Wax
A thin layer of wax improves the appearance of many stones, particularly those that are hard to polish. This includes jade, carvings with hard to reach areas, and soft stones like turquoise and lapis. The wax sometimes has a dye added to enhance the color of the gem as well.
A hot point will tell you if a stone has been coated with wax. The procedure is similar to testing for plastic. Heat the hot point to a dull red; however, this time try not to touch the stone, just hold the hot point very close, about 1/16 of an inch or so. Observe the surface of the stone. If it is coated, you will see the wax liquefy and bead up like perspiration.
Do not use heavy liquids before hot point testing. In fact, it is not a good idea to use heavy liquids on any porous stone because it will sweat out with the heat from the hot point and it is very toxic to breathe.
Acid Testing
| AAA Always Add Acid to water! Never the other way around!If you pour water in acid, it will boil and splash out of the container. The acid will burn metal, wood, skin, etc. So always remember the AAA rule. |
These organic materials are carbonates, as are malachite and rhodochrosite. You can make a positive identification of them by testing their reaction to hydrochloric acid. Of course, this is dangerous to both the tester and the stone, so you need to exercise great care!
First,, you need to obtain your acid. If you do not have ready access to a chemical supply house, you can purchase muriatic acid at any building supply. It is the same acid and is used to clean concrete.
| Necklace AlertAny time you are testing a strand of beads, make sure you do not get any fluid on the cord.The acids used for identification and the liquids we use for dye testing, will damage the cords. |
Wear plastic gloves while handling acid and mix your acid in a glass jar with a glass lid, because metal lids will dissolve from the vapors. Get all your materials together before opening the container then, work quickly, cover both containers, and get back inside. Remember to label the bottles clearly and store them somewhere safe (i.e. out of the reach of children, where a roaming cat will knock them over, etc.).
Testing Procedures
Now, after all those precautions, we can get to the testing. Place a very small drop, no larger than a pinhead, in an inconspicuous spot. Again, this is usually near the girdle. Now observe the reaction. If the test material is a carbonate, the acid will effervesce. The reaction happens quickly, so wash the stone under running water as soon as possible. Since you are using such a small amount, low power magnification is helpful in seeing the results.
One type of imitation lapis lazuli, not the glass or plastic types, is distinguished by acid testing as well. You know that the white spots in lapis are calcite. Light colored lapis is a result of fine particles of calcite throughout the lazurite. To differentiate between imitation lapis and natural lapis, perform the acid test as above. If the stone is natural, you will note a rotten egg smell, hydrogen sulphide, and it may effervesce if enough calcite is present. In the imitation version, you will also note the rotten egg smell, but no effervescence and the acid will leave a white spot.
Dye Tests
Gems are frequently dyed to enhance their color. Being able to distinguish between natural and enhanced stones is essential to the gemologist. The materials used will vary with the gems you are testing, but the procedures remain essentially the same.
As always, you need to choose an inconspicuous location for your testing. See the Necklace Alert above for more details. Use a cotton swab to test the stone. Clean the stone immediately with a damp cloth, and then inspect the swab for dye.
Lapis lazuli is one of the most common recipients of dyes. Any dark lapis is suspect and needs testing. The same holds true for turquoise and many other soft, porous gems. Often dye can be distinguished with a microscope. See “Identifying Inclusions” in our Reference Library for more information on distinguishing dyes with a microscope. You should always use the microscope first, before resorting to a destructive test. The dye test information given here is for cases where the microscope examination is inconclusive.
Dye Testing
To distinguish a dyed stone, dip your cotton swab in acetone. Rub it briefly in an inconspicuous area and then clean the stone. Now check your swab. If there is a colored residue on the swab, you can be certain it is either a dyed natural gem or an imitation.
If the swab is clean, you could still be looking at dyed natural gem, but one with a wax coating. Now is the time to use a hot point test to see if the gem is waxed.
Acetone is the most useful substance for testing, but it will not pick up all dyes. If your swab comes clean, repeat the test with denatured alcohol and again with hydrochloric acid. Only then can you say with assurance the gem is naturally colored.
Dyed Black Pearls
The above test is occasionally recommended for black pearls; however, natural black pearls fluoresce to long wave UV, where dyed ones do not. So UV testing is preferred as there is no danger to the pearls.
Difficult Separations
( This is Premium Content. You are a Gold Member and therefore are viewing this entire article.)
This is Premium Content. You are a Gold Member and therefore are viewing this entire article.)
by Don Clark IMG CSM
While
some gemstones are fairly easy to separate, others are more difficult,
presenting a greater challenge to the individual attempting the
separation. Two of the most difficult separations to make are scapolite
from citrine and apatite
from tourmaline. The good news is that there are tricks you can use to
tell them apart. These tricky separations both begin with a thorough
examination.Scapolite and Citrine
Say you were given a light yellow gem to examine. You did a basic RI reading, yielding two readings on the table 90 degrees apart. The results were 1.545 and 1.552. The polariscope tells you that it is uniaxial.
You put this information in Gemology Tools Pro; Color: yellow, Optic character: DR U, and the high and low RI. You get a long list of possible matches using this information, so you move on to other tests to aid you in your separation. Other easy-to-find information is the UV reaction and pleochroism. First, you find the stone is inert to UV, both long and short wave. That applies to both species, so it is no help. You check for pleochroism and find it is weak, pale yellow to yellow. Now the only stone that meets these characteristics is quartz.
The near hits do not include scapolite because the pleochroism for scapolite is moderate to strong. If your eyes are fatigued it is easy to make this kind of mistake, especially if you are looking at two shades of the same hue.
Another mistake is to assume that you have measured the full birefringence of the stone. Scapolite almost always has more birefringence than quartz. Had you been confused as to its identity, measuring the RI on another facet would have given you the necessary information.
In many cases, scapolite will fluoresce to UV. Since quartz is always inert, that is a quick separation. Citrine will often show a bullseye optic sign. That is also an identifying feature. So learn to look for the easy information first and do not take anything for granted.
Apatite and Tourmaline
Here is another tricky situation. You are examining a blue gem and your basic RI readings are 1.635 and 1.638 and the optic sign is DR U. Put this and the color in Gemology Tools and you get another long list of possible gems. As you examine their properties to find things that will distinguish them, you find that almost all the properties are shared. One clear separation is the specific gravity.
So you add the SG of 3.26 to your query. Now your data shows varieties of tourmaline as the clear choices, with one rare gem and apatite as near hits. If you were to assume indicolite is the gem, you could well be wrong. Also, because specific gravity readings are notoriously inaccurate on small stones, you need to consider this if you are working with a small stone.
While it is time-consuming, you need to find something else to make this identification. One of the best separators is to test the RI on another facet. As with the example of the scapolite, you probably have not measured the full birefringence of the stone. Apatite has a maximum birefringence of .008 and tourmaline is always higher than that.
Another easy test is for pleochroism. Apatite is blue and yellow. Tourmaline can have a variety of pleochroic colors, but never blue and yellow.
These are the two most missed identifications on our practical exams. I hope this information will help you not just identify these particular gems, but serve as an example on how to be thorough in your examinations.
How Gems are Identified Part II: Deductive Reasoning
( This is Premium Content. You are a Gold Member and therefore are viewing this entire article.)
This is Premium Content. You are a Gold Member and therefore are viewing this entire article.)
by Don Clark IMG CSM
Learning the ArtOne of the biggest mistakes that beginning gemologists make is relying strictly on their data. After making some initial tests, they begin looking at long lists of potential species and get lost. It is easy to lose your way in a sea of data and important clues are often overlooked, which is why identifying gems is considered an art. Gemology is as much art as it is science and relying strictly on your analytical skills can be a handicap. Taking accurate readings is essential, but so is applying your reasoning ability. This is why deductive reasoning is one of the most important skills you can develop.
| Material | Hard High | Hard Low | SG High | SG Low | RI High | RI Low | Birefringence | Optic Sign | Dispersion |
|---|
| MALACHITE | 4.5 | 3.5 | 4.05 | 3.60 | 1.909 | 1.655 | .254 | B- | - |
|---|
| SIDERITE | 4.5 | 3.5 | 3.96 | 3.83 | 1.873 | 1.633 | .240 | U- | - |
|---|
| SMITHSONITE | 5 | 4 | 4.45 | 4.30 | 1.848 | 1.621 | .227 | U- A | .037 |
|---|
| AZURITE | 4 | 3.5 | 3.89 | 3.30 | 1.846 | 1.63 | .106 – .110 | B+ A | - |
|---|
| RHODOCHROSITE | 4.5 | 3.5 | 3.70 | 3.40 | 1.840 | 1.574 | .201 – .220 | U- A | - |
|---|
| EPIDOTE GROUP | 7.5 | 5.5 | 4.20 | 3.10 | 1.830 | 1.640 | .004 – .049 | B+/- A | .019 – .030 |
|---|
| TOURMALINE GROUP | 7.5 | 7 | 3.90 | 2.82 | 1.820 | 1.604 | .006 – .080 | U- | .017 |
|---|
| UNAKITE | 7 | 6 | 3.20 | 2.85 | 1.760 | 1.520 | - | A | - |
|---|
| PARISITE | 4.5 | 4.5 | 4.36 | 4.25 | 1.757 | 1.676 | .081 | U+ | - |
|---|
With the advent of Gemology Tools, a searchable database program for gemstone identification, searching data is easier and errors are minimized. All you have to do is enter the correct information and let the computer search the data for you. This is a wonderful asset for the gemologist as it removes the tedium while minimizing errors.
The
Gemology Tools software allows you to enter a number of different
measurements so you can identify any gemstone quickly and easily.
Traditional gemology teaches that, when examining an unknown gem, the two most useful pieces of information are the refractive index (RI) and specific gravity (SG). While an RI reading is fairly quick and straight forward, SG readings are time consuming and often inaccurate. Gemology Tools allows you to search by visual properties as well as measured data. That means that, with only a visual observation, an RI and polariscope exam, you can begin your search. Time-consuming specific gravity readings and huge reference charts are now a thing of the past.
Below is an outline of testing procedures that emphasizes deductive reasoning while incorporating modern identification techniques. This will speed up your identifications and make them more accurate.
Testing Procedures
Step 1: Clean the Gem
The first step in the identification process is to clean the gem thoroughly. Lint often resembles surface scratches when examining a gem through a loupe or microscope, so keep a small artist’s brush handy. If you see scratches on the surface, try to brush them off.
Step 2: Examine with Loupe
Next, examine the stone with a loupe and an overhead lamp. You will see much more with a loupe than with a microscope, because of the constantly changing relationship between the stone to the light. Inclusions that are easily missed with the fixed lighting of a microscope will suddenly appear, then disappear again. This is oblique lighting taken to the extreme and is a very useful technique.
Examine the stone from every direction, constantly holding the stone under the light. Look inside and look at the surface. Give it as thorough an examination as possible.
Look for the following information. Write down everything you see. You never know what will be important later.
Cut
Is the gem well cut, or does it look like something a student was practicing on? Look for discipline in the cutting, meaning do the facets meet at well defined points and are all the facets in a tier the same size? If it is a cabochon, watch the light pass over the surface and note if the movement is smooth and even, or if it snakes across the surface.
Also note the polish. The surface may be mirror like, or it may dull. You may find it is pitted, even with an otherwise good polish. On some gems you will find the polish does not cover the whole facet. Note all of these facts.
Color
Note everything to do with color. This includes not only the hue, but the zoning and pleochroism as well. These are very important in identification as well as value.
Inclusions
Inclusions are some of your most important clues in gem identification. Some are identifying in and of themselves, while others make important distinctions between species or between natural and synthetic.
Even if you are not sure what you are seeing, make a note of it. This is another case where you do not know what may be important later.
Physical Characteristics
Note any physical characteristics you may see. This includes any fractures and whether they are straight or curved, large or small, few or numerous. Note any little chips on the culet or edge. You may need the microscope to see them clearly. If so, just note their position for now.
You may also find damage to the gem. This has little to do with identification, but it can have a lot to do with value, so make a note of it.
Step 3: Microscope Examination
If you did your first examination carefully, there are only a couple of things to check. The first thing to inspect are the inclusions. On stones where they are important to identification, you may need to use moderate to high magnification and a variety of lighting techniques to see them clearly.
Next, turn the stone sideways and check to see if it is an assembled stone. You do not need to do this if there are inclusions that run nearly top to bottom; however, if the stone is lacking inclusions or if it only has a couple small inclusions that may be bubbles, it could be a doublet.
You can still miss doublets unless you immerse the gem. This is messy and cleaning the stone is time consuming, so it is not a standard procedure. Your safety net is that Gemology Tools will let you know if your unknown may be an assembled stone. If your other data doesn’t eliminate the possibility of an assembled stone, you will have to go back and use the immersion technique.
If you have not already, look at the fractures. See if they are something other than conchoidal.
Write down your observations as you go along.
Step 4: Check for Color Change
Color change is defined as the difference between what you see in natural light versus incandescent. You will usually see the change between incandescent and fluorescent, but rarely between natural and fluorescent.
You need to check every stone for color change. Whether this becomes a separate step depends on what lighting you use in your initial examinations. You are using three light sources throughout the course of your examination: the room lighting, an overhead lamp with the loupe, and the one in your microscope. If one of these is natural or fluorescent and the other incandescent, you have it covered. All you need to do is to pay attention to what you have seen. If not, you will have to take another step to determine if the color is stable or changes.
Step 5: Deductive Reasoning I
During your initial examination you are likely to see important clues. Here are some of the most important ones and what you can deduce from them.
Color
If you see strong zoning, you are most likely looking at quartz, sapphire, or tourmaline. This is not a property that is well defined for all gems and many exceptions exist. Still, this is a good clue that the stone you are examining is most likely quartz, sapphire, or tourmaline.
If you see strong dichroism, you are most likely examining a tourmaline. If the dichroism is two entirely different hues, it is almost certainly tourmaline. You will find strong dichroism in other gems, but tourmaline is by far the most common.
Bicolored gems are usually tourmaline or sapphire. Some Australian sapphires are yellow and blue. Fluorite is another gem that comes with many colors, but it is rarely used in jewelry. By far the most common bi or multi-colored gems are tourmaline.
If you are examining a colorless gem, the most important thing is to determine if it is a diamond or not. Diamonds usually have distinctive visible properties. If you find them, your testing is over.
Inclusions
You need to become familiar with all types of inclusions. See Identifying Inclusions and Synthetic Gemstones and Their Identification. Learn these by heart, because they are some of the most important pieces of information you can obtain.
Here are some of the most common inclusions and what you can glean from them.
Crystals almost always mean a natural origin.
Photographs of crystal inclusions in natural gems.
Round bubbles are usually seen in synthetics, especially glass and plastic.
Curved striae are indicative of flame fusion.
If the stone is loupe clean it is most likely a synthetic. Natural gems are occasionally loupe clean, so you cannot rule them out, but the chances are very good that you have a synthetic.
Trigons and bearded girdles are indicative of diamond.
Trigons are typically found in diamonds.
Large rutile crystals are common in quartz and tourmaline, while microscopic rutile crystals are seen in corundum.
Lily pad inclusions are only found in peridot.
Stress fractures are common in heat-treated corundum and are fairly easy to recognize.
Silk is found in garnet and corundum; however, each gem species has a unique pattern.
Hollow growth tubes are only found in beryl, tourmaline, and spodumene. The latter is rarely found in jewelry and RI quickly separates beryl from tourmaline. That does not mean you will not see spodumene, just that it is unlikely.
Hollow growth tubes are only found in beryl, tourmaline, and spodumene.
Horse tail inclusions are found in demantoid garnet.
Cut
As a general rule, sloppy cutting is only done on cheap stones. Better cutting indicates a valuable stone. There are exceptions to this, but it is a worthwhile assumption in the early part of the identification process.
A straight fracture is perfect cleavage. Since only a limited number of stones have perfect cleavage, this can be helpful in your identification.
Lots of small, internal fractures may indicate perfect cleavage. If the gem you are examining has a lot of small, internal fractures, check to see if they are in parallel rows.
Also examine any chips on the surface. There are usually some tiny ones near the culet or girdle and you may need a microscope to see them well. If they are conchoidal and have a vitreous luster, they are insignificant; however, anything else is also an important clue.
Polish Luster
Most of the stones you examine will have a vitreous luster. This means little in identification because it is so common; however, if you see anything else — greasy, waxy, etc. — it is an important clue. You cannot assume anything from this, but as you go further along the identification process, it will eliminate several possibilities.
Making Assumptions
The above information will allow you to make assumptions about the stone’s identity. It is important to remember that assumptions do not prove anything, but they are helpful in your learning.
The assumptions you make are also a good way to double check yourself. For example, if the gem you are looking at is bicolored, pink and green, and your data tells you it is an apatite, you better think again. Tourmaline and apatite have nearly identical properties, but only tourmaline is bicolored pink and green.
Further Testing
Step 6 Refractive Index
The next step is to take a basic RI reading. That is two positions on the table 90 degrees apart, noting the highest and lowest RI.
To determine the RI, use a refractometer and take two readings: one along the long axis and one along the short axis.
Step 7 Polariscope Testing
In most cases, your RI readings will tell you if a gem is doubly refractive. When you think a stone might be singly refractive, a quick check in the polariscope is necessary.
While you have your polariscope handy, you can typically find the optic sign with a minimum amount of effort as well as check for pleocroism. These are useful clues and it is worth the minute or so that it takes to check.
Step 8 Search Database
At this point, enter the data you have collected into Gemology Tools for a list of possible gem species.
After entering your information, the software will search for gems that match your criteria. From the results, you can compare the properties of the possible species. Look for the properties that are shared by the fewest stones. Measure one of those properties, enter it in Gemology Tools and do another query.
In many cases, that will be enough to make a positive identification. If not, you will have to look for another property to measure. Continue this process until you only have eliminated all but one species.
Special Cases
Rare gems
Any time you come up with a rare gem you need to be especially careful. The gem may be something exceptionally rare, but it is also very likely you made an error. Go back and double check all your information and make sure you entered it correctly in the search fields.
Since most rare gems are soft, look at the facet edges. Soft gems will have rounded edges. If they are sharp, the gem is at least somewhat hard. Poor cutting can give you rounded edges as well, so be careful in what you deduce.
No Inclusions
Clean gems are the most difficult to identify. There are a few natural gems that are clean or that have inclusions that are so small you cannot see them with 40 power magnification; however, a gem that clean is almost always synthetic. Since there is so much at stake, you cannot risk a guess.
On these rare occasions, you may need 80 or 100X magnification to make a proper identification. Typically, though, you usually find useful inclusions with enough effort. Take a handheld light and shine it through the stone from several directions, while watching through the microscope and inspect the gem from several positions. This will almost always turn up something useful.
Step 9: Deductive Reasoning II
Now stop and see if your results make sense. Go back over your initial observations and compare them to your findings. Any discrepancies need to be accounted for. Either your observation or your readings are inaccurate and you need to know which one it is. This is one of the most important elements in learning gemology, so do not take it lightly.
You might have seen what you thought was a heat treatment halo, but were not positive of it. If you came up with a sapphire, you can rest comfortably with your results. Look at the inclusion again and impress it in your memory. If, on the other hand, you came up with garnet you had better recheck your results. You could well have mistaken anomalous double refraction for the actual thing.
If you come up with a rare gem like pyroxmagite, you always need to be cautious of your results. You could be looking at a very rare gem, but in most cases you are not.
If your results tell you the stone is peridot, but the color is pink, you can be sure you made a mistake. Go back and verify your information until you find an error. See if that change leads you to a correct answer. If it does not, look further until all the information concurs.
These types of mistakes are common, which is why we recommend that you take your time. If you are unsure of something, look it up. If you are tired, take a break. And always, always, double check yourself. Whether you are taking an exam or dealing with a customer, telling someone their pink stone is a peridot is inexcusable.
Examples
While preparing this lesson, I had the following stones to identify. These were not made up as examples, but were actual gems that needed identifying. However, they perfectly illustrate the procedures and problems you will encounter.
Example 1
This green gem looks like tourmaline.
Tourmaline Samples
With the loupe, I see that it is well cut. It has some fingerprints and fractures, but no identifying inclusions.
The end facets are black and no light is passing through them. This is called a “closed C axis” and is common to tourmaline, but little else. For our purposes we will simply note that it has strong pleochroism.
Next I took a basic RI reading. On the long axis it measured 1.643. Turning it sideways, it read 1.641.
I put it in the polariscope, which verified that it was doubly refractive, but it did not show any stress. Since I thought I had enough information to prove it was tourmaline, I did not make much effort to find the optic sign, but went right on to my database search.
In Gemology Tools I entered the following information:
Color: green
Transparency: transparent
Pleochroism: strong
Luster: Vitreous
RI High: 1.643
RI Low: 1.641
Optic Character: DR
To my surprise, I got four possible species: apatite, asparagus stone, viridine, and tourmaline. All but viridine have the same optic sign and their specific gravities overlap. I looked further and noticed the only separating factor is birefringence. That made sense, as tourmaline has a very high birefringence, .018 to .040, and I had just measured a tiny bit.
I went back to the refractometer and tested another facet. I got a high RI of 1.651 and entered that in Gemology Tools. This time when I did a search I got just tourmaline.
The total time on this identification was about 10 minutes.
Example 2
Example 2 is a transparent blue gem.
With the naked eye, I can see that it has nice color, is well saturated, and just slightly greenish. It is factory-cut with a big window and has some long, straight inclusions.
Example 2 gem has some internal fractures along with growth tubes.
I entered the following information into Gemology Tools:
Color: blue
Optic Character: DR
RI High: 1.583
RI Low: 1.569
Transparency: transparent
Luster: vitreous
Magnification: hollow growth tubes
A search gave me only one possible gem, aquamarine. That made the identification complete and it only took about 5 minutes.
Example 3
The next gem was a white opal in a nice gold setting. It actually has a lot more fire than this photograph, but nothing extreme.
Opal can be identified by eye –since no other gem looks like it. Since this is a white opal, there is no need to check for enhancements, like soot or sugar treating. The only step to a complete identification is to put in in a microscope and check for the chicken wire effect.
The total time for this ID was about 3 minutes
Example 4
Example 4 is the blue center stone in an expensive-looking ring.
Center Stone
The loupe reveals that this has a rich blue color, it is transparent, and well cut with sharp facet edges. This is just what you would expect from a high-end gem. Based on the anticipated value, color, and sharp facet edges, I assume it is a sapphire.
There is no pleochroism visible. Inside I can see zoning and some needles, but a microscope is needed to see the more clearly.
A loupe view of Example 4 reveals no pleocroism, some zoning, and some needles.
A microscopic view of Example 4 shows that the zoning is straight and the needles are red.
Next I entered the following information in Gemology Tools search engine:
Color: blue
Optic Character: DR
RI High: 1.774
Transparency: Transparent
Magnification: straight growth lines
I got assembled stones, benitoite, and sapphire as results.
I know this is not an assembled stone, because the inclusions run up and down through it. Benitoite has more dispersion than a diamond at .044. If it were a benitoite, I would have seen that. So the answer is sapphire.
Again, the identification took about five minutes.
Side Stones
One of the ring’s side stones as seen through the loupe.
Some of my observations were unusual though. The gems were all loupe clean and have very high dispersion. This is possible with very fine diamonds, but they were only pretty well cut. The facet edges were somewhat sharp, but not what you would expect from diamond.
A positive identification is not possible with the stones in the setting, but they are probably CZs as moisanite is usually distinguishable by its color and doubling, and GGG and YAG do not have this much dispersion. Glass, zircon, and other diamond substitutes do not come close to this appearance, so they are most likely CZs.
Example 5
Example 5 is an eye clean, yellow stone in a ring setting.
Before I picked up the loupe I was impressed with the setting. It is very heavy, with a white gold head and a yellow band. This is clearly an expensive piece.
The loupe revealed a couple fine, fingerprints that were only visible from side. The stone showed strong doubling. No pleochroism was observed. It has a good commercial cut, with sharp facet edges and good meets, but lots of chips and scratches in the polish.
The loupe view of example 5 reveals a few fine fingerprints and shows strong doubling in the stone.
I took my RI readings and got 1.620 and 1.618. These readings do not match with the strong doubling I observed. Like the tourmaline above, I am working almost directly on an optic axis. I know birefringence is much higher than .002.
Polariscope testing was a bit tricky as the setting is closed under the stone, but since I had so little information, I wanted the optic sign. There was a bit of strain, but it was hard to find the optic sign. I finally found it through the table by holding the ring at a 45 degree angle. The gem is uniaxial.
I was now able to enter the following information into Gemology Tools:
Color: orangish yellow
Optic Character: DR U
RI High: 1.620
RI Low: 1.618
Polish Luster: Vitreous
Transparency: Transparent
The search results gave me three possibilities: assembled stones, calcite, and tourmaline.
I went back to my microscope to see if the fingerprints went from the top to bottom, but I could not tell anything for certain. That meant I needed to do an immersion test.
This ring was so large, I had to use a water glass to submerge the entire thing. Fortunately, we had some olive oil that was beyond its peak, so it was used. I was able to tell the stone was whole, that it was not assembled.
That left calcite and tourmaline. Calcite is very soft, with a hardness of only 3. The facet edges were too sharp for it to be calcite, so I ended with a positive identification of tourmaline.
This identification took about 15 minutes, with a good part of that spent cleaning the oil off the stone.
Conclusion
You need to remember that gem identification is as much an art as it is a science. Your reasoning powers are every bit as important as your ability to take readings and look up data.
It takes a lot of experience to become familiar with the gems and the testing procedures. To accompany this lesson, we have a series of What is it? quizzes. These give examples of other practical identification problems. Together, they will give you a good idea of how to precede with your identification problems.
Identification of Synthetic Diamonds
( This is Premium Content. You are a Gold Member and therefore are viewing this entire article.)
This is Premium Content. You are a Gold Member and therefore are viewing this entire article.)
By Donald Clark CSM
Synthetic
diamonds are not new. General Electric has been making them for a half
century. Most of the diamond abrasives we use are GE synthetics.
However, it cost more to create a gem grade diamond than to mine them,
so few have ever been produced.In the Summer of 2003, two new synthetic diamonds were introduced. While the manufacturers are reliable in making their disclosures, some resellers are not. Knowing that most small diamonds never go to a lab, many have been sold without disclosing that they are synthetic.
CVD Diamonds
New on the market are Chemical Vapor Deposition diamonds. They come in sizes to 3 carats, with a normal range of inclusions.
Identification
Identifying CVD diamonds is farily simple. They have a unique strain pattern that does not resemble natural diamonds and strong red fluorescence. In addition, they lack the typical “Cape Line” at 415 nm. Instead, they present a strong line at 737 nm.
HPHT
GE developed the high pressure/high temperature method of growing diamonds. A new company called Gemisis has improved the process and is producing gem grade diamonds. It takes them longer to produce a colorless stone than a yellow one which is worth considerably more, so currently the production is limited to canary yellow gems.
These gems were released in the Summer of 2003 and are now in full production. We have no reports on the size range, but they tend to be in the higher clarity grades. Even though Gemisis is only selling yellow stones at the moment, be aware that other colors and colorless diamonds are also being made by the HPHT process.
Identification
Symmetry
With rough diamonds, one quick clue is the perfect symmetry to the rough crystals. You rarely find perfect form in natural gems, much less a lot of them.
Magnification
Like the GE product, Gemisis diamonds are grown in a metal solvent and particles of metal are captured in the diamond matrix. Called flux, it is dark gray or black. It closely resembles natural inclusions and the key to separating them is with overhead lighting. If synthetic, a metallic luster will be seen in reflected light.
Crystal inclusions, whether diamond, garnet or zircon, are proof of a natural growth environment.
Natural diamonds may exhibit planar bands of color, which are distributed irregularly throughout the stone. Synthetic diamonds show distinct color zoning with highly colored and pale colored areas separated by sharp boundaries. It often forms geometric patterns such as squares, octagons, and cross shapes. In most cases, this is just indicative of a synthetic and not proof of origin. However an “hourglass” pattern proves synthetic origin.
To observe color zoning in colored diamonds, immerse the stone in a high RI liquid and use diffused light in your microscope.
Graining in synthetic diamonds can also can create a “stop sign” or “hourglass” pattern. This is easiest to find when looking through the pavilion of a faceted stone using a combination of dark field illumination and fiber-optic lighting.
Ultraviolet
Identifying synthetic diamonds requires that the stone be viewed from all angles under both wavelengths of UV. Natural diamonds commonly fluoresce blue to long-wave and a weaker yellow to short-wave. In contrast, synthetic diamonds typically fluoresce yellow to yellowish green to both wavelengths of UV. The reaction is often stronger to short wave than long, or it may be equally intense to both.
In making the separation, the pattern or zoning of the fluorescence is more important than the color itself. The internal growth sectors of a synthetic diamond generally produce geometric, (octagonal, square, etc.) or cross-shaped patterns of fluorescence. Fluorescence is evenly distributed in natural diamonds. Low magnification can be helpful in detecting the patterns.
The UV test is particularly useful for parcels of diamonds.
Spectrum
Synthetic diamonds lack the traditional cape line. Natural type Ib and type II diamonds also lack a cape line, so that is not proof of origin. However, type Ib and type II diamonds are quite rare, so it is a good clue that you may have a synthetic.
Most synthetic diamonds are type Ib or IIa. Natural type Ib or IIa diamonds typically do not show spectral lines in the visible range of the spectrum. Some synthetic diamonds do show a series of absorption lines between 470 and 700 nm. Seeing several of these lines would indicate synthetic origin.
Magnetism
Too much has been made of synthetic diamonds being attracted to a magnet. The test is difficult to perform and it proves nothing.
To make this test you need to suspend the diamond from a string, in location that is protected from air currents. Bring a strong magnet close to, but not touching, the stone and move it from side to side. If the diamond is attracted, it will move in a parallel motion. However, lack of a magnetic response does not prove it is a natural. If an HPHT diamond contains just a small amount of flux inclusions, it may have no response to even a very powerful magnet.
A few natural diamond may contain inclusions that could cause the diamond to show a faint attraction to a strong magnet as well. However, they will normally show enough other evidence to prove their origin.
Strain
In a natural diamond, strain is visible as bright interference colors that generally occur in banded, cross-hatched, or mottled patterns. In contrast, the strain in a synthetic diamond mainly appears black and gray and shows a cross-like pattern. This again is not proof of origin, but combined with other information it may be enough to complete an identification.
NPD Diamonds
Synthetic nano-polycrystalline diamonds, (NPD,) are the newest synthetics on the market. They are created in a multi-anvil press by a sintering process which converts graphite to diamond at 15 GPa and 2300-2500°C. So far, only brown diamonds are created by this method.
Identification
NPD diamonds show a roiled appearance when examined under magnification. They also have a distinct, patterned, red luminescence.
The absorption pattern of these stones show lines at 612 and 668nm.
Identifying Inclusions
( This is Premium Content. You are a Gold Member and therefore are viewing this entire article.)
This is Premium Content. You are a Gold Member and therefore are viewing this entire article.)
by Donald Clark CSM
Magnification
opens the door to a fascinating new world in gemology. Some of the
marvels you see rival the gem itself for beauty and intrigue. On a
practical level, a study of inclusions will tell you a lot about a gem.
For example, inclusions reveal how the gem was formed, which
distinguishes between natural and synthetic stones. In addition, some
gems have specific inclusions that their look alikes do not, which helps
with your identification. Other inclusions are unique and identifying
in themselves.When working with magnification, the gemologist needs to pay careful attention. It is easy to mistake what you see, so you have to be cautious not to jump to any conclusions when identifying inclusions.
One of the greatest challenges is getting a sharp focus. Knowing how to use your microscope and the various lighting techniques is essential. (See Using the Microscope.) Still, it is challenging and sometimes impossible to get a small detail in perfect focus.
This is why discretion is essential. For example, can you distinguish the inclusion pictured below? It is difficult to tell if the ends are terminated, or if they are bubbles. These are actually crystals from inside a garnet. Mistaking them for bubbles would be disastrous, as crystals indicate a natural gem and bubbles a cheap synthetic.
Can you identify this inclusion?
It takes practice to distinguish between some of these inclusions. These pictures and descriptions are just a guide, the actual learning comes from experience. Look at as many gemstones as possible. Examine them with your loupe and then with a microscope. Keep practicing until you are an expert at finding inclusions and identifying them.
The variety of inclusions is so large that a complete listing never be accomplished. This article lists the primary ones needed for identification purposes. They are arranged in four categories: Inclusions Found in Natural Gems, Inclusions Found in Synthetic Gems, Inclusions Found in Enhanced Gems, and Inclusions of Specific Gems.
Langganan:
Postingan (Atom)
