Gems - tourmaline

Gemstones, prodigally gifted with radiance and color, seem to stand apart from the common workings of nature. The ancients thought gems were of supernatural origin and ascribed to them all manner of powers. According to the Greeks, a goblet of amethyst could ward off drunkenness; the Burmese believed that a ruby sewed into the flesh would make a warrier invulnerable.

By the 17th Century, scientists had come to understand gems as "rare and noble productions of nature," in the words of the English physicist Robert Boyle. They are distillled from duller rock by processes that often involve...

Tourmaline is one of the prize lithium-based gemstones. Tourmaline belongs to the trigonal crystal system and occurs as long, slender to thick prismatic and columnar crystals that are usually triangular in cross-section. The style of termination at the ends of crystals is asymmetrical, called hemimorphism. Small slender prismatic crystals are common in a fine-grained granite called aplite, often forming radial daisy-like patterns. Tourmaline is distinguished by its three-sided prisms; no other common mineral has three sides. Prisms faces often have heavy vertical striations that produce a rounded triangular effect. Tourmaline is rarely perfectly euhedral. An exception was the fine dravite tourmalines of Yinnietharra, in western Australia. The deposit was discovered in the 1970s, but is now exhausted. All hemimorphic crystals are piezoelectric, and are often pyroelectric as well.