World Of Gems Conference 2017 Proceedings Book

WORLD OF GEMS CONFERENCE V - 28 - SEPTEMBER 2017 BLACK GEMS be considered as the key detection method. Features such as girdle condition, facet edges, luster and surface condition of the gems must be carefully examined to establish if the material is black diamond. The following gems are commonly sold as black diamond. • Black Moissanite • Black CZ • Black Spinel • Black Sapphire Naturally colored black diamonds (overly dark green) make up a very small portion in the market. Most black diamonds are treated for color (Figure 2). The most attractive and homoge- nous material is created from gray diamonds that are colored by dense clouds. The heated black diamonds are created from any type of highly included diamond by the formation of graphite around inclusions and in fractures. Graphite forms in the voids around the particles by heating in an inert atmos- phere (Figure 3). Heated black diamonds are more frequently encountered than irradiated black diamonds. Detection of ir- radiation is difficult, sometimes impossible. The most effi- cient irradiation to create black color is neutron bombardment; such stones can be radioactive after the treat- ment, especially when they are highly included. Most imitations of black diamond are reasonably easy to identify once the basic observations mentioned above are applied. However, black moissanite and black spinel create a challenge since they are OTL. Unfortunately, majority of gemologists assume that black spinel has the same RI of gem quality transparent spinel and do not realize that spinel is a solid solution series mineral which includes OTL black spinels (Figure 3). Black moissanite on the other hand is assumed to be identi- fied with a diamond-moissanite tester. Contrary to a common misconception, both black diamond and black moissanite are electrically conductive, therefore the tester fails to distinguish between them. The following points should be carefully as- sessed while testing; - Some black moissanite will display green or blue color when checked with strong fiber optic light, so do irradiated black diamonds. - Other black moissanite is totally opaque and evenly colored, so do some irradiated black diamonds. - Some black moissanite will look like an opaque polycrystalline material with grains and graphite inclusions etc., which may resemble heated black diamond. - Irradiated black diamond might fluoresce green, although mostly such gems are completely inert; some of the heated black diamonds show green and/or blue fluorescence. Black moissanite is typically completely inert. CONCLUSION Black gems are not considered of high value when popular gems are encountered. Therefore, once the identity is con- cluded as a non-diamond material, value becomes less of an issue. Gemologists should never underestimate going back to basics in testing of black materials, especially microscopy. It is important to understand the variety and amount of pro- duction of the materials in the market. In the case of black diamonds, unless advance testing techniques such as Raman and Infrared spectroscopy is used, it is not possible to posi- tively identify the material. u FIGURE 2. Black diamond and gold cuff from Fur and Claw Couture Collec- tion of Bear Brooksbank of London, uK. Photo by Tom Mannion. FIGURE 3. Graphite inclusions in heated black diamond. Courtesy of Thomas Hainschwang.