Bortz

1906 September, X-Rays and a Diamond Thief, Popular Mechanics, page 932, It is also noticeable that the black diamond, No. 20, a dull and lusterless substance, is as transparent to the X-rays as the brilliants, while the diamond bortz, No. 6, is much less transparent than either, though more so than the imitation, No 19.

These main categories of industrial diamonds, gem stones, bortz, and carbonados, are further subdivided into smaller classes. For instance, particularly fine bortz of a round form composed of crystals radiating from a common center are known as ballas.

A large part of the production of the Belgian Congo is crushing bortz.