What is Opal?
Opal is a semitransparent mineraloid. Unlike the crystalline forms of silica, it is composed of hydrated silica (SiO2-nH2O), and its water content varies from three to 21%, by weight. In nature, it’s found as irregular veins, in masses, or in nodules in fissures or cavities in nearly any type of rock substrate, including limonite, sandstone, rhyolite, marl, and basalt. Most commercial opal comes from Australia, and some comes from Ethiopia, in Africa.G14
Opal diffracts light, causing its highly varied coloration that includes white, gray, yellow, red, magenta, orange, rose, green, pink, brown, slate, black, olive, and blue. In addition, it may be colorless or opaque to semitransparent. Gemologists refer to common opal as “potch,” and it does not exhibit the colors seen in precious opal. Opal’s opalescence, or colors resulting from highly dispersed light, results from its lattice structure of tiny silicon dioxide spheres that are closely packed in planes.
Although lapidaries do not valuate potch like opal gemstone, potch, too forms beautiful masses, and some forms of non-gemstone quality potch are especially attractive. For example, Mexican fire opal (yellow, orange, and red), opalized wood, resin opal (honey-yellow), menilite (brown or gray), hyalite (colorless, transparent), milky raw opal (white with blues or greens), geyserite, diatomite, and some opal-matrix combinations make especially attractive cut stones.G15
Of these, fire opal, a transparent to translucent opal, with its warm colors and occasional green flecks, has become especially well known, as has the Mexican jelly opal. Gem cutters often use both in their host material, producing a cut stone known as “cantera opal.” Cantera’s a Spanish word referring to, “quarry stone.”
Aside from natural opal, manufacturers synthesize artificial opal, which gemologists can discern from natural material by its regularity and lack of fluorescence under ultraviolet light.
Through history, opal has been traded under many animated names, including Pandora, Light of the World, and Empress. During the Middle Ages, opal, with its rainbow of colors in a single stone, was considered a gem capable of carrying the qualities of the whole spectrum of gems whose colors were represented in the stone.G16 It has even been credited with the ability to confer invisibility!
With so many colors, that all look different from any particular angle, and its unusual translucence still inspires mystery and magic. It is definitely one of the most variable precious stones, in terms of its colors and degree of translucence. Its variability comes from its physical properties and structure, and schillers, or reflected color flashes within the opal, make it unpredictably colorful and brilliant.G17