Opal

Australia has been the world's primary source since its mines began commercial production in the 1890s. Lightning Ridge (New South Wales) is famed for black opal; white opal comes from White Cliffs (New South Wales) and from Mintabie, Andamooka and Coober Pedy (South Australia); boulder opal is mined only in Queensland. Ethiopia (near Wegel Tena in Wollo Province, and Shewa Province), Mexico (fire opal from Querétaro) and Brazil are also important sources. Other deposits are in Central Europe, Honduras, Indonesia, Madagascar, Peru, Turkey and the United States.

Opal is amorphous, with a low refractive index (1.37–1.47), no birefringence and a low specific gravity (about 2.15). Precious opal shows play-of-color; sphere sizes of about 0.1 micron produce violet and about 0.2 micron produce red. Main trade types: white or light opal (light bodycolor), black opal (dark bodycolor), fire opal (transparent to translucent brown, yellow, orange or red bodycolor, often without play-of-color, also called "Mexican opal"), boulder opal (with fragments of host rock, called matrix), and crystal or water opal (clear background).

Opal may be impregnated with oil, wax or plastic. Opal doublets and triplets are thin slices of opal glued to a base material, triplets being covered with a clear quartz dome.

With a Mohs hardness of 5 to 6.5, opal can be scratched by diamond, ruby, sapphire, emerald and many other gems, so store it separately. The safest way to clean it is with warm, soapy water; other methods may damage the opal or any filler. Prolonged exposure to water can weaken the adhesive in doublets and triplets, and even natural opal can fracture with high heat or sudden temperature changes.

The Romans gave the gem the name opalus, synonymous with "precious stone"; the name is believed to come from the Sanskrit upala, "precious stone", as India was the source of the first opals brought to the Western world. Around 75 AD the Roman scholar Pliny wrote that opal combined the colors of ruby, emerald, topaz, sapphire and amethyst. Admirers gave famous opals names such as Pandora, Light of the World and Empress. Australia became the main source after its mines began commercial production in the 1890s.

Opal is the traditional October birthstone and the gem for the 14th wedding anniversary. In ancient Rome it symbolized love and hope, and Europeans have long considered it a symbol of hope, purity and truth.

Because opal shows the colors of other gems, the Romans regarded it as the most precious of all, and hundreds of years ago it was believed to embody the virtues of all colored stones. Bedouin and Arabic legends said opal held lightning and fell from the sky during thunderstorms. The ancient Greeks believed opals gave their owners the gift of prophecy and guarded them from disease, and it was once believed to preserve the color of blond hair. The superstition that opal is unlucky comes from Sir Walter Scott's 1829 novel Anne of Geierstein, not from ancient belief; through most of history it was regarded as a lucky gem. These are historical beliefs, not established properties.