Opals are a semi-precious gemstone composed of silica and water. Water combines with a certain kind of silica and drips into cracks within the Earth. Over a period of millions of years, as the water alternately drips and evaporates over the silica, the material hardens and begins to take on color, which can range from white to blue, green, red, orange and black, with white and green colors being the most common. The iridescence of opals is due to the semi-transparent silica spheres which both reflect and refract light at various wavelengths of the spectrum; opal is capable of refracting every color of the rainbow.
The most prized opal colors are black opals from Australia which display a dark blue-black body color with intense flashes of rainbow. Milk opal or white opal is the most common, featuring a white body color with multi-colored flashes of rainbow. Fire opals are yellow, orange and red and are typically semi-transparent with little to no iridescent color play, although strong flashes of green may be observed. The largest producer of opal in the world is Australia which supplies 97% of the world's opals. Opal is October's birthstone.
See related: Birthstones, Matrix, Cabochon