Precious Opal

One of the few non-crystaline gemstones, Opals get their iridescence and fire from the uniformity of the positioning of the tiny silica spheres of which it is formed. It is usually cut as cabochons, often with other rock left on the back if the vein is too thin to provide its own strength.

See also Matter, Gems, Si and Garnets Feldspars.

— Scotus - 20 Jan 2002