International Gem Society
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MODERN DETERMINATIVE GEMOLOGY
... One of the biggest challenges for the gemologist is, not just obtaining accurate data, but properly analyzing it. After making some initial tests, they begin looking at long lists of potential species. It is easy to loose your way in a sea of data and important clues are often overlooked.
by Don Clark IMG CSM
|Material ||Hard |
|Birefringence ||Optic |
|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, searching data has become much easier and errors are minimized. All you have to do is enter your 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.
...As nice as this is, that is just the icing on the cake. The data searching features of Gemology Tools are so powerful you can do things never before possible. The International Gem Society has developed new procedures to take advantage of this technology. This simplifies the identification process, reduces the amount of lab work required and the time involved, while reducing errors. That is a significant advancement from one single piece of inexpensive software.
...Traditional gemology teaches that when examining an unknown gem, the two most useful pieces of information are the refractive index and the specific gravity. While an RI reading is fairly quick and straight forward, specific gravity 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 referencing huge charts are now a thing of the past.
Step 1, Clean the Gem
...The first step in any identification is to clean the gem thoroughly. Lint closely resembles surface scratches, 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 over head 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 it 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.
...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 these facts.
...Note everything to do with color; not just the hue, but zoning and pleochroism as well. These are very important in identification as well as value.
...Inclusions are some of your most important clues in gem identification. Some are identifying in and of themselves; 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 of where you do not know what may be important later.
...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 haven't 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 verses 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; the room lighting, an overhead lamp with the loupe, and the one in your microscope. If your room lighting is natural and another 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 Refractive Index
...The next step is to take a basic RI reading. That is two positions on the table at 90 degrees apart, noting the highest and lowest RI.
...You do not need to use the polarizing filter unless you get the same reading in both positions. That would indicate a singly refractive gem, which is an important clue. If your results still indicate a singly refractive gem, you will need to confirm that with a polariscope.
Step 6 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.
...You can also find the optic sign on many stones with a minimum of effort. Pleochrosim is also sometimes easy to see in the polariscope. These are useful clues and it is worth the minute or so it takes to check for them.
...Write down everything you see.
Step 7 Search Database
...At this point, enter your data into Gemology Tools. It has fields to enter data, like high and low RI, optic sign (which may be nothing more than singly or doubly refractive,) and specific gravity. It also has fields to enter your observations. These include color, pleochroism, polish luster, transparency, and most of the inclusions you found with magnification.
...After entering your information, it will do a 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.
... While preparing this article, 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.
... This stone looks like a tourmaline. Part of that is based on the shape; tourmaline crystals are elongated, so rectangular gems are common. The color is green, but slightly grayish. It is certainly not an emerald or other chromium colored gem.
... 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, 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 didn't make much effort to find the optic sign, but went right on to my database search.
... In Gemology Tools I entered the following information:
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 all their specific gravities overlap. I looked further and noticed the only separating factor 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.
... This is a transparent, blue gem. With the naked eye, I can see that it has nice color; well saturated and just slightly greenish. It is factory cut with a big window and has some long straight, inclusions.
... As I examined it with a loupe, the inclusions jumped right out at me. Most of them are internal fractures, but a few are clearly parallel growth tubes.
... This told me it was either a tourmaline or an aquamarine. I took a basic RI reading and determined that it was doubly refractive. I did not bother with the optic sign, as that appeared to be unnecessary.
... I entered the following information into Gemology Tools:
Optic Character: DR
RI High: 1.583
RI Low: 1.569
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.
... The loupe reveals that this has a rich blue color, it is transparent and well cut with sharp facet edges. There is no pleochroism visible. Inside I can see zoning and some needles, but a microscope is needed to see them more clearly.
... Under 20X with back lighting, I could tell that the zoning is straight - these are not curved striae. The needles were a bit of a surprise, as they are red.
... Next I took a RI reading and measured 1.774 on the long axis. I couldn't get a reading on the short direction as it wouldn't sit flat on hemisphere. All I could get from the polariscope was that the stone is doubly refractive.
...Next I entered the following information in Gemology Tools search engine:
Optic Character: DR
RI High: 1.774
Magnification: straight growth lines
... I got assembled stones, benitoite, and sapphire for my 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.
... This ring features a big yellow stone that is eye clean. 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 microscope didn't reveal anything new.
... I took my RI readings and got 1.620 and 1.618. This doesn't accord 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. Since I had so little information, I wanted the optic sign. I 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
... The search results gave me three possibilities: assembled stones, calcite and tourmaline.
... I went back to microscope to see if fingerprint went from the top to bottom, but I couldn't 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 submerse the entire thing. Fortunately, I 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.
... The search features of Gemology Tools allow for easier and more accurate identifications than ever before. It has many other software modules that are also very useful, but outside the scope of this article.
... Gemology Tools is available through the IGS Book Store, or you can get it with a substantial discount with a new membership.
... It takes a lot of experience to become familiar with the gems and the testing procedures. There is a longer and more detailed version of this article for IGS members. To accompany this lesson, we also 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.
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