Diagnostic Gemology
Whether you are a gemologist, a doctor or any other scientist, it all comes down to having empirical proof to back up your diagnosis. Phrases like “it could be…(diamond)”, or “should be …(diamond)”, or “I guess it’s…(diamond)”, are better not be used. These phrases usually imply that you most likely have doubt of your findings but it doesn’t mean that you are a bad gemologist. When this is the case, it would be wise to consult a colleague gemologist to ask for second opinion, seek for literature or refer to a lab with advanced research technology that can help you make the right diagnosis.
When determining a gemstone, it’s important to understand the difference between indicative and the diagnostic features. It’s not uncommon that a gemstone has been determined based on only indicative features, which could lead to a wrong diagnosis. This could be due to time pressure or just performing careless research. When it comes down to diagnosing whether the gemstone has been treated or not, even more care has to be taken. On the question how many diagnostic features are necessary to determine a gemstone, the answer is simple: as many as you can find but preferably nothing less that THREE.
Take for example a red gemstone showing a ruby spectrum on your first gemological test. If the gemstone ought to be determined only on this single diagnostic feature, it would be a mediocre manner of research with risks involved. In this case, you are still required to take 8 refractive index readings on two facets, one on the crown and one on the pavilion side. After it would be logic to take specific gravity test. By now you would have 3 diagnostic features. Next step would be researching the gemstone on microscope to separate natural from synthetic and to verify whether it had been treated or not.
Question: Is this a complete research report?
Answer: No, but it is acceptable.
Next question: What’s the need to execute all other tests if there is already a diagnostic ruby spectrum on the first test?
Answer: There are different reasons, but the simplest one would be in case the ruby would be a doublet it could still show a normal ruby spectrum, but have an aberrant specific gravity.
Foremost, to avoid all these problems, just research with a very strict WORKSTRUCTURE and CONSISTENCY. Don’t skip any steps along the way. Secondly, your results will depend on your stored knowledge.
Third and not to be taken lightly, consider the QUALITY of your gemological equipment and accuracy of the reference materials – like identification tables, manuals etc…
As gemology stands for a non-destructive way of researching gemstones, the approach of your research could affect your results as well, especially when dealing with mounted gemstones.
Diagnostic Versus Indicative Features
It’s rather rare to deal with one a single diagnostic feature that will allow you to determine the gemstone with one single gemological test. Therefore it’s required to execute several different gemological tests. An easy way to remember the difference between indicative and diagnostic features is, (1) when you have a certain amount of indicative features which all point out to the same gemstone, (2) there are NO other possible gemstones that would have ALL the same indicative features, then those features would be considered as diagnostic for THAT gemstone. Bear in mind that there are some specific diagnostic features for some gemstones, e.g. horse tails in demantoid, Bulls eye in quartz, tabby extinction in synthetic spinel etc….
A Proposed Work Structure
- A thorough examination with a 10X gemological loupe/hand lens. Generally speaking it will reveal quite a bit of information, depending on your experience. After executing this step, an ADVANCED gemologist could already have an idea which type of gemstone/imitation he is dealing with. This would allow him to execute specific tests enabling him to find the specific diagnostic features to make the right diagnosis. A BEGINNER should not use this method and follow the same work structure over and over without skipping any steps along the way. In other words the gemstone should be tested with all classic – conventional gemological equipment. Once having all the test results, then only a beginner should select the diagnostic features and highlight them on the lab/research report. Don’t forget to verify that you are not dealing with a doublet or triplet.
- Quickly take a refractive index reading on one facet. This is a useful step to decide which immersion liquid would be necessary to observe the gemstone in the immersion cell for polariscopy or later for microscopy.
- Take 3 specific gravity tests and make an average of those results. Some of these results can be very specific (e.g. Diamond, Quartz, Danburite, Corundum, YAG, etc..).
- Have a thorough look in the immersion polariscope to separate uniaxial from the biaxial gemstones and to determine the optic sign of the uniaxial gemstones being positive or negative. In other cases, the gemstone can be isotropic, polycrystalline or show anomalous double refraction.
- Make a thorough calculation of the RI, 8 readings on 2 facets, one on the crown side and one on the pavilion. In case you are sure of the stone being uniaxial +/- through polariscopic research, then 4 readings are sufficient. Beware that birefringence could be less accurate with only 4 readings. Don’t forget to turn the polarization filter when using spot reading/distant vision method. Some gemstones have a very big birefringence so the second RI reading could be higher than 1,81. In case the refractive index doesn’t measure higher than 1,81 you now have a collection of 3 important indicative features that would become diagnostic if they point out to the same gemstone. If these 3 features still indicate other possible gemstones, continue with further gemological tests to rule out all other possibilities and zero in unto one diagnosis.
- When the refractive index is higher than 1,81 use a Gemmeter, Duotester or Diamond tester. It’s advisable to only use the results of the Gemmeter or Duotester as indicative feature.
- Check whether the gemstone has a specific spectrum with a spectroscope. Apply green filter if necessary.
- Use a dichroscope to verify whether the gemstone is dichroitic or trichroitic.
- Check the gemstone with Ultraviolet long and short wave. It could reveal important information on a gemstone being synthetic. In some cases UVL and UVS miscroscopy can be useful to observe internal growth structures or treatments. Beware that this can be hazardous to health.
- Research the gemstone on microscope, probably the most demanding technique of all. Use crossed polarizer’s, immersion cell, dark field, color filters, shadowing, different sort of lighting techniques, etc… Basically everything that is needed to find diagnostic features for the gemstone being natural or synthetic and whether it had been treated or not.
- Research all test results with accurate gemological identification tables or any necessary needed literature.
- Most important of all, write down your diagnosis with the correct NOMENCLATURE, this can alter slightly, depending on the place/country where you had your training. Example, in Europe, it would be according to CIBJO.
There are different methods/work structures that can be used to determine a gemstone, but experience will show what is suitable for you.
A Few Tips
- Never let the price of the gemstone or jewelry affect your research.
- Always start your research from scratch, don’t get influenced by hearsay information, this could be misleading.
- Don’t diagnose a gemstone based on test results coming from another person.