Natural Glasses

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No account of the silica gems would be complete without mention of the amorphous impure natural glasses found on the earth surface in a few areas. The most common of these obsidian, a name derived from the Latin word for it, obsio.

It is a translucent to opaque material, usually black but sometimes grey or brown, formed by the rapid cooling of volcanic lava. It often contains inclusions of gases and minerals which may give it a silvery sheen, a spangled effect, a chatoyance, or the semblance of white snowflakes on a black background. It usually contains bubbles. In jewelry it is formed into brooch cabochons, pendants or beads, its low hardness precluding it from exposure to hard wear. It occurs in many parts of the world and was vitally important raw material from which neolithic peoples made knives, scrapers, arrowheads and other tools as well as objects and figurines for ceremonial use.

Complete mystery still surrounds the origin of another group of natural glasses, the tektites (from the Greek for 'molten'). Volcanic and meteoric provenance has been postulated for them but no satisfactory theory yet exists. They are named after the localities in which they are found, as australities, indochinites and billitonites, for example.

The only variety that has found some use in jewelry is moldavite, named after a river in Bohemia. It is olive- to bottle-green, full of curious swirls and bubbles; it is usually cut with facets. Equally mysterious is the origin of the yellow, cloudily transparent silica glass found in the Libyain-Egyptian desert in 1932. None of the natural glasses are of much value since few are sufficiently attractive and rare, and none are hard enough to make good gemstones.