Green Opal "smaller" hand stones from Madagascar.

The mineraloid opal is an amorphous variety of quartz, SiO2 nH2O, hydrated silicon dioxide with a water percentage of sometimes as much as 20%. Opal can be colourless, white, milky blue, grey, red, yellow, green, brown and black. Often many of these colours can be seen immediately, caused by interference and diffraction of light passing through minute, regularly arranged gaps within the microstructure of the opal, known as the Bragg lattice. These gaps become filled with secondary silicic acid and form thin lamellae within the opal as it hardens. The term opalescence is used to describe this unique and beautiful phenomenon, although play of colour would be a better term. Opal is one of the minerals that form or can replace fossils. The resulting fossils are treasured by collectors. "Opal" is derived from the Old Indian word úpala', meaning gemstone. Distinctions are made between the glittering precious opals, the yellow-red fire opals and the common opals. Their physical properties show important differences. Opal is the official gemstone of South Australia. Opal has been cut as a gemstone since ancient times. Initially it was used for the production of primitive tools, later for ornamental objects. Opal was already used by the Assyrians, Babylonians and Romans. Precious opal is one of the most mysterious stones. Traditionally it was considered a stone that brings bad luck, especially for people who did not clean the stone properly. The precious opal described by Pliny comes from the Dubník deposits in Slovakia, which were already being mined at that time. This stone was attributed with magical powers. It helped against melancholy, calmed the nerves, healed the heart, protected against worries, restored eyesight and gave the eyes a new shine. In addition to naturally occurring opal, opal is produced in many different ways, experimentally and commercially. The resulting material can be distinguished from natural opal by its regularity; under magnification the patches of color are seen to be arranged in a "lizard skin" or "chicken wire" pattern. Synthetic opals are further distinguished from natural opal by the former's lack of phosphorescence under UV light. Also, synthetic opals are generally lower in density and are often highly porous; some can even stick to the tongue. Two notable manufacturers of synthetic opal are the Kyocera and Inamori companies of Japan. Most so-called synthetic opals, however, are more correctly called imitations, since they contain substances not found in natural opal (e.g., plastic stabilizers). The Gilson opals often seen in fine jewelry are actually imitations consisting of laminated glass with scattered bits of foil.