Natural-Color Nonconductive Gray-to-Blue Diamonds

Two new categories of gray-to-blue diamonds are described. First, two type IIb blue crystals with what appears to be a type IIa outer skin were not electrically conductive in the rough, but conducted electricity once faceted. Second, some nun-electrically conductive grayto-gayyish blue diamonds from the Argyle mine, Australia, were determined to be type la and to contain unusually high amounts of hydrogen. This latter type of coloration, not previously reported for diamonds, may be due to hydrogen-related defects in the crystal structure.