Equal to the task
Here are the steps I take to identify coloured gemstones and diamonds in my store:
1) Visual observation with the naked eye—observe the stone’s colour, hue, tone, saturation, surface lustre, brilliance, chips, and abrasions along the facet junctions. Each piece of information gathered can help bring you closer to identifying the subject stone correctly.
2) Look at the stone through magnification, preferably with a high-quality research-grade gemmological microscope with dial zoom magnification. Depending on the task at hand, loupes are not always effective. Lighting is also a consideration. Overhead reflective lighting allows you to observe surface features, lustre, hue, tone, saturation, and surface abrasions along facet junctions, the girdle, and culet. You should be able to determine the colour of the abrasions, as well as their structure (e.g. grainy, flat, or sharp edges).
Dark-field illumination allows you to determine whether the subject stone is transparent, translucent, semi-translucent, or opaque. Colour zoning, growth patterns, curved internal lines, included crystals, and air bubbles can also be observed. Direct high-intensity lighting reduces surface glare and flaring, and can help highlight certain internal features from oblique angles.
3) Testing the subject stone’s refractive index (RI) is a must. This is usually the quickest and easiest way to help identify it, since RI is unique and can instantly eliminate a lot of possibilities.
4) Check for birefringence to eliminate stones from your list of possibilities. For example, red garnets are single-refractive, while rubies are double-refractive, yet they can be mistaken for one another due to their similar RIs. To test for birefringence, rotate the subject stone 1/12th of a turn to the right and record your reading. Do this until you have rotated it a full 180 degrees. When following this process, you should have six sets of high and low RI readings when the stone is double-refractive and only six constant single readings when it is single-refractive.
5) Test the subject stone under both long-wave and short-wave ultraviolet (UV) light within a fully enclosed UV viewing cabinet. Diamonds and some gemstones fluoresce different colours and with varying intensities under long-wave and/or short-wave UV light. Keep in mind, however, applying UV light in this manner should never be used as a standalone definitive test for any stone—it is merely an additional test. Consider placing known test stones inside the UV box so you instantly know whether your long-wave and short-wave lights are working properly every time you use them.
6) Always prove what the subject stone can’t be, rather than trying to prove what it might be. Use simple deductive reasoning and logic in your identification process.
7) Having a gemmological reference library close at hand is a must. Every professional gemmologist should have one and be adding to it as the latest research findings or new information on treatments becomes available. Never rely solely on your mind and memory. Double check your facts and conclusions against your reference books.
