Gem Focus – December 2020

Posted on December 8, 2020 by The Gemworld Staff

BLACK DIAMONDS:
THE DARK SIDE OF THE MOON

Black gems tend to be ranked towards the lower end of the value scale by the gem trade. However, thanks to increasing interest in unusual gem materials, accompanied by advertising, they are receiving newfound recognition. Modern designers are successfully integrating a number of unusual natural black materials, especially black diamonds, into their work, many of which have not been customarily used in the trade before. While designers enjoy new options, due to value considerations, identification challenges are numerous for the gemologists. It is important to understand the variety and amount of production of the materials in the market.

The common identification methods and tools are typically inadequate for detecting majority of black gem materials, black diamonds being the most challenging. However, gemologists should never underestimate the fundamentals of gem testing of black diamonds, especially microscopy. The conventional testing tools such as diamond/moissanite tester or determining the RI, SG, or spectrum might be insufficient but, since the goal is to eliminate the non-diamond material, getting back to basics is necessary. Microscopy should be considered 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 1). The most attractive and homogenous 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 and around the particles by heating in inert atmosphere (Figure 2). Heated black diamonds are more frequently encountered than irradiated black diamonds.

Figure 2. Heat treated black diamonds.
Photo by Gary Roskin.

Detection of irradiation is difficult, sometimes impossible. The most efficient irradiation to create black color is neutron bombardment; such stones can be radioactive after the treatment, particularly when they are highly included. 

Most imitations of black diamonds 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, the 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 such as chromite and magnetite. (GemGuide subscribers can access The Black Gem Challenge, by Cara Williams of Stone Group Labs., published in the May/June 2013 Gem Market News here.)

Black moissanite on the other hand is assumed to be identified with a diamond-moissanite tester. Contrary to common misconception, both are electrically conductive, therefore the tester fails to distinguish between them. The following points should be carefully assessed while testing;

  1. Some black moissanite will display green or blue color when checked with strong fiber optic light, so do irradiated black diamonds.
  2. Other black moissanite is totally opaque and evenly colored, as are some irradiated black diamonds.
  3. Some black moissanite will look like an opaque polycrystalline material with grains and graphite inclusions etc. which may resemble heated black diamond.
  4. Irradiated black diamonds might fluoresce green, although most are completely inert. Some of the heated black diamonds show green and/or blue fluorescence. Black moissanite is typically inert.

Figure 3. Black diamond and gold cuff from Fur and Claw
Couture Collection of Bear Brooksbank of London, UK.
Photo by Tom Mannion

Practicing gemologists are fully aware that for positive identification of black diamonds along with detection of treatment, advance testing techniques such as Raman and infrared spectroscopy should be utilized. In most cases, once the identity is concluded as a non-diamond material, value becomes less of an issue. Therefore, the final call is made once the value vs. cost of detection balance is established.

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