Orange Opal, from Owyhee/U.S.A.

The mineraloid opal is an amorphous variety of quartz, SiO₂·nH₂O, hydrated silicon dioxide with a water content of up to 20%. Opal can be colourless, white, milky blue, grey, red, yellow, green, brown, or black. Often, many of these colours can be seen immediately, caused by the interference and diffraction of light passing through minute, regularly arranged openings within the microstructure of the opal, known as the Bragg lattice. These openings become filled with secondary silicic acid and form thin lamellae within the opal as it hardens. We refer to this unique and beautiful phenomenon as opalescence, but a more accurate term would be play of colour. 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 'upala', meaning gemstone. Distinctions are made between the shimmering 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 worked as a gemstone since ancient times. Initially, people used opal to manufacture primitive tools, and later, they used it 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 brought bad luck, especially for people who did not clean it properly. The precious opal described by Pliny comes from the Dubník deposits in Slovakia, which were already being mined at that time. People attribute magical powers to this stone. It helped with melancholy, calmed the nerves, healed the heart, protected against worries, restored eyesight and made the eyes shine again. Apart from its natural occurrence, various experimental and commercial methods produce opal. The resulting material can be distinguished from natural opal by its regularity; under magnification, patches of colour are seen in a "lizard skin" or "chicken wire" pattern. Synthetic opals are further distinguished from natural opals by the former's lack of phosphorescence under UV light. Furthermore, 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. However, most so-called synthetic opals are more correctly called imitations since they contain substances not found in natural opal (e.g., plastic stabilisers). Gilson opals, often seen in fine jewellery, are actually imitations, consisting of laminated glass with scattered bits of foil.