It may have a few flashes but is valued for its color and clarity. Common opal is sometimes used as the backing material for more desirable varieties of more precious opal. Australia may be the world's most important source of opal, but it is also found in Mexico, Brazil, Hungary, and in the US, Nevada, California, and Idaho. Opal is a low temperature mineral, found in cracks or cavities filled after the host rock was formed or as replacement after fossils or wood. It is usually found in sedimentary rock, such as sandstone or claystone, and igneous rock, such as rhyolite.
Mining is usually a small scale operation, with hand-operated machinery and tools.
Opal Doublet, taken from Gem Hut . As the water content diminishes, the stone may lose color and crack. In addition to assembled doublets and triplets, imitations and synthetics are found. Opal and tourmaline are the commonly accepted October birthstones. It is usually fashioned in a cabochon cut and set in rings, earrings, pendants, or pins.