Natural Opal Information
Opal is a mineraloid gel which is deposited at relatively low temperature and may occur in the fissures of almost any kind of rock, being most commonly found with limonite, sandstone, rhyolite, and basalt.
The water content is usually between three and ten percent, but can be as high as 20%. Opal ranges from clear through white, gray, red, orange, yellow, green, shore, blue, magenta, rose, pink, slate, olive, brown, and black. Of these hues, the reds against black are the most rare and dear, whereas white and greens are the most common; these are a function of growth size into the red and infrared wavelengths. Common opal is truly amorphous, but precious opal does have a structural element. The word opal comes from the Latin opalus, by Greek opallios, and is from the same root as Sanskrit upala[s] for "stone", originally a millstone with upara[s] for slab. Opals are also Australia's national gemstone.
Opal is one of the mineraloids that can form or replace fossils. The resulting fossils, though not of any extra scientific interest, appeal to collectors.
Unlike other gemstones opal does not have a crystal structure, and the texture consists of amorphous silicon dioxide having submicroscopic silica spheres. Depending on the size and how these silica spheres are arranged together determines how they diffract light. These silica spheres have diameters that range from 0.15 to 0.30 microns (one thousandth of a millimeter ) which is a little smaller than the wavelength of visible light which ranges from 0.4 to 0.7 microns in diameter. In order for opal to be considered precious opal the silica spheres must be uniform in diameter and arranged in neat rows. A precious opal that has flashes of red will also contain all the other colors of the spectrum as the opal is seen by all other angles. It is interesting to know that this structural color of opal is produced by light, conveyed by the eye and is then understood by our minds.
Black Opal is that naturally occurring one piece or solid opal, which is jet black to dark grayish-blue or deep brown in color, and absorbs most white light impinging on it and reflects only a minimum. As a consequence, all optical diffraction effects are much more brilliant because of the sharp tonal contract.
Black Opal, a gemstone which has had an important effect overseas as a product of Australian, requires this precise meaning so that the quality of this gem can be meaningfully established. Sometimes off-colored white opal has been passed off to a visitor as being black opal!