Gemstone of the Week: Black Diamonds
Everybody loves a beautiful brilliant colorless diamond. You will find those in many engagement rings, wedding bands, earrings, and used as accent stones in all sorts of fashion jewelry. However Black diamonds have been introduced into the fine jewelry market. Although they have a dark semitransparent to translucent appearance, they have a sparkle that can be seen across a room just like a colorless diamond.
Natural black diamonds are usually colored by inclusions of sulfides, graphite or other mineral inclusions or, in rare cases, hydrogen-related clouds that extend throughout the stone. Treated black diamonds are generally heavily fractured stones that are subjected to high-temperature, low-pressure treatment, which graphitizes the fractures, turning them black. Artificial irradiation can also produce a dark enough color so as to appear black, but this is less common (H. Kitawaki, “Gem diamonds: Causes and colors,” New Diamond and Frontier Carbon Technology, Vol. 17, No. 3, 2007, pp. 119–126). Natural radiation stains provide an important clue to the color origin of a diamond. Diamonds that have been naturally colored by radiation can have stains associated with that color, though this is not a definitive criterion. In this case, the stains are so numerous and thick that they cause the diamond color. A close-up view of the pavilion (figure 3) shows the relative transparency of the diamond contrasted with the highly saturated color of the radiation staining of the fractures. The coverage of the stains is so complete that only a few transparent areas canbe seen in the face-up view. Some of the color was due to the presence of small black inclusions, but most was due solely to the natural radiation stains, an unusual cause of color.
Because of the clarity characteristics and the structure of black diamonds, they can be difficult to cut into fancy shapes, such as tapered baguettes. For this reason, black diamond tapered baguettes and marquises are rarely used in jewelry.