Basics

Gemological analysis is a form of investigation: we collect clues, sort them, analyze them and arrive at an identification.

To do this, we must follow a logical path. Depending on the school, this path will be different. For example, at the GIA, the manual for determining gems starts with the color of the stone, which gives a first sorting. The second step is the measurement of the refractive index. This gives several potential stones and the corresponding complementary tests. At Gem-A, we first analyze the stone with the naked eye, then with a 10x magnifying glass and only then do the tests (polariscope, refractometer, etc.) must be logical. If we practice a test, it is by knowing why this one and not another. It must make it possible either to reduce the number of possibilities, or to positively conclude the identity of a stone. For example,it is unnecessary to use a Chelsea filter on a colorless stone, since it will not reveal any chromogenic agent, whether it is chromium, vanadium or cobalt.

The very first thing to understand when starting to analyze stones is that one should always make a positive identification of a clue ! This is crucial! Most beginners who have a choice between two possible identities, conclude with the identity of a stone by saying that “since I did not see this there, then it cannot be this stone.” Therefore, it can only be the other alternative ”Nothing is further from the truth! If we do not observe alternation of ignitions and extinctions with the polariscope, it is possible that we look at the stone along its optical axis. On the other hand, if we observethis alternation, we can put aside all the gems of the cubic system and the microcrystalline gems! In the first case, we didn’t see anything and we deduced something wrong. In the second case, we cannot deny what we have seen and which makes it possible to reduce the possibilities or to conclude. Always make a positive identification of a clue.

Let us now see in detail the path that I personally use.

First of all, I look at the gem with the naked eye. This step is essential, in particular in the case of crudes. We will first look at what is on the surface: breaks, polishing, shine, chipping, etc. All of this gives us initial clues and guides our subsequent analyzes. For example, we observe a pale blue stone with a greyed leaf and heavily striped faces. These two elements should lead us to conclude that the hardness of this stone should not be very high. If we add to this that this stone is very bright with significant fires, these indices lead us to verify two things: the presence of doubling of the edges seen with a magnifying glass and the refractive index greater than the capacity of the refractometer. If these last two elements are confirmed, then we will be in the presence of a blue zircon.It is the concordance of all these clues that leads to the conclusion. If we only had low hardness and a vitreous luster as a clue, then it might as well be glass, kyanite or benitoite.

Then, still with the naked eye, I observe the interior of the stone, its transparency, its color zones, its color schemes, the doubling of the edges, etc. We regularly observe straight growth streaks with very specific angles between them. This automatically excludes materials such as glass, microcrystalline materials, etc. If we are in the presence of a light olive green stone with a bold luster of which we can see a doubling of the edges with the naked eye, it is not useful to continue further: we have between the hands a nice peridot.

Let us dwell on the rough for a moment. The observation of a fairly well formed crude makes it possible to define its crystalline system . This step will save considerable time for the rest of the analysis. In the case of rough rolls, it is much more difficult. In general, we practice a window in the rough, that is to say that we polish a small area in order to be able to look inside the stone. This also makes it possible to measure its refractive index with a refractometer.

The next step in gemological analysis is observation with a 10x magnifying glass. In the case of small stones, we immediately look under a magnifying glass. The observation with a magnifying glass follows the same path as the observation with the naked eye: first the exterior of the stone, then the interior. The magnification makes it possible to capture details invisible to the naked eye, such as the shape of small fractures (conchoidal, granular, etc.), cleavage planes, scratches on the facets or the table, scratches on the leaf, small inclusions, etc. We note everything we see and it is interesting to make a sketch to position these elements. This will make subsequent searches faster.

After having had a first idea of ​​the stone I have between my fingers, I use the polariscope. This allows me to know if the stone is isotropic, anisotropic, amorphous or microcrystalline. Isotropic stones are of the cubic system. No matter how you look at them with a polariscope, they stay dark without any change in intensity. Warning: you must observe a gem along several axes and rotate it a full turn for each observation. Anisotropic gems turn on and off alternately twice over a full rotation.

We change the position stone and start the observations again. An amorphous material like glass will exhibit a reaction that has nothing to do with the previous two. Instead of turning on and off, we can see dark waves coming and going in the stone. Finally, microcrystalline materials stay on continuously when rotated. You can use the conoscope to locate the optical axes of the gem.

Depending on the result of the polariscope test, I am looking for a pleochroism with a dichroscope. Anisotropic stones often exhibit a certain pleochroism. In some cases, it is so strong that it can be seen with the naked eye, such as in the case of tanzanitis. Note that I first practice the polariscope test before that of the dichroscope because some stones, although anisotropic, have an almost zero pleochroism. The pleochroism of very light stones is particularly hard to see.

The next test is the refractive index (IR) measurement. A refractometer is used for this. It has become an indispensable instrument for the gemologist. Refractometers can be found on the market from 100 €, but they are often poorly calibrated and not very bright. I made the mistake of buying one on startup and found it had an index offset of 0.02 which is huge. A good refractometer is very expensive, but will last for years if taken care of. A great alternative is to buy a used one from a good brand. Here is the one that I have been using for several years now and which has the great advantage of having internal lighting that works on AA batteries. You can find it here for less than 400 €.

Measuring the IR makes it possible to greatly reduce the number of potential identities of our gem. The refractometer also makes it possible to determine if the stone is isotropic or anisotropic (but we already know this thanks to the polariscope), if it is uniaxial or biaxial, and its optical sign + or -. It is one of the gemologist’s most practical tools. It is not for nothing that schools like the GIA make this measurement the very first step in the analysis. For my part, I prefer to do it a little later because in the field I do not always have the possibility of measuring the IR (go and try to ask a salesperson for this at a trade show…). On the other hand, after calm analysis, it suffices to bring the stone back to the seller.

In case the IR of the gem exceeds the capabilities of the refractometer (1.80), I use the grating spectroscope. It is an extremely practical instrument that is half the size of a pen. Its only downside is that you have to exercise regularly for it to be effective. We must immediately know how to recognize the spectra observed. This takes a little time and practice as the spectrum is not always easy to see, especially for light stones. I have to admit that I hated using it until my gemology teacher showed me how to do it effectively. After an afternoon, I was won over and it is a test that I now practice systematically in order to keep an eye on it. It is almost essential to differentiate certain garnets and certain zircons.

I use the microscope very last. First, because a close examination with a magnifying glass will have already given many clues. Secondly, because it is very bulky. My microscope is part of my portable laboratory and I have to assemble and disassemble it after each use. It remains almost essential to identify the treatments. The microscope also makes it quite easy to spot the real pearls (fine or cultured) of their main imitations.

I deliberately omitted the use of the Chelsea filter. What was true yesterday for this filter is no longer true today, and will be different tomorrow. Some genuine emeralds do not react pink to the Chelsea filter. Other gems react similarly to a real emerald. For my part, I only use this filter during batch analysis to verify that all the stones in a batch show the same reaction. If this is not the case, it is because we are faced with a batch of non-homogeneous stones. It can be interesting to look at blue stones because often synthetic stones like blue spinel are very bright red.

At each stage of the gemological analysis, we note our observations. Each of them leads us to suppress potential identities. A mantra to repeat all the time: always make a positive identification of a clue.