Opal has the same chemical composition as quartz, but contains from 1 to 2 percent water, and is not crystallize. It has a certain type of regular structure, but no at the atomic level, being composed of alignment of tiny spheres which form a compact, three-dimensional net worth. The name opal is apparently derived, through the Greek opallios, from the Sanskrit upala, meaning “precious stone”.



Appearance:
It occurs as narrow veins of upto ten centimeters or more, or as nodules, inside cavities or cracks in silica-rich rocks. It may also found psedomorphous after other minerals. It may have a whitish to light green, pale green, sky-blue, smoke grey, black, yellow to orange or reddish background color. It can be semi opaque, with a vaguely porcellainlike appearance or similar of broken glass with shiny, conchoidal fracture. It is more often translucent and milky with an appearance that is so characteristic, it is described as opalescent. Opals can even be fully or largely transparent; such stones are usually orange-yellow to red in color. The most highly priced variety display internal iridescence due to light diffraction by network of tiny spheres of which they are composed. These types are collectively known as noble opal or precious opal. The range of colors apparently depends upon the size of the spheres, or rather, the distance between the rows. In gem quality, precious opal, three sets of distinctions are made. The first, according to the brown color of the material, distinguishes light or white opal to dark or black opal. The second, applied to each of these two varieties, is based on the range of colors in the iridescent patches; and the third is based on the size, shape, and distribution of the patches. The transparent or semi transparent, no iridescent variety (known as common opal) is also used as a gem if it is attractively colored, because of it’s orange-yellow to reddish orange color, it is known as fire opal.



Occurrence:
One area of eastern Czechoslovakia formally belonging to Hungary has been mining opal since roman times and the only source of noble opal for Europeans until the nineteenth century. Nowadays, most opal comes from Australia, where the finest quality opals are found. Other sources are Mexico and, to a lesser extent, Guatemala and Honduras. Low-value or sub gem quality varieties are found in many other places, especially the United States and Iceland.



Simulants and Synthetics:
It was long considered impossible to imitate opal. In recent years, however an imitation, which, at first sight, looks deceptively similar to opal, has appeared on the market. It is called “Slocum stone,” after its inventor. A plastic imitation also recently comes on to the market. It is very similar indeed to the natural stone, being composed of microscopic spheres, like copper. Fortunately, its low melting point, hardness, and density can distinguish it, despite its appearance of its iridescent patches is fairly distinctive, but it can nonetheless only be distinguished by an expert.