tourmaline

1. (mineralogy) A complex black or dark-coloured borosilicate mineral, compounded with various chemical elements and considered a semi-precious stone.Examples: "The floor of the chamber was tesselated, of marble and green tourmaline, and on every square of tourmaline was carven the image of a fish: as the dolphin, the conger, the cat-fish, the salmon, the tunny, the squid, and other wonders of the deep."; "Although there are dozens of different types of gems, among the best known and most important are […] . (Common gem materials not addressed in this article include amber, amethyst, chalcedony, garnet, lazurite, malachite, opals, peridot, rhodonite, spinel, tourmaline, turquoise and zircon.)"countableuncountable