Pliny, the Roman philosopher and writer, called Opal a gemstone which combines the best possible characteristics of the most beautiful of gemstones: the sparkle of Almandine, the purple of Amethyst, the golden yellow of Topaz, and the blue of Sapphire, "So that all colours shine and sparkle together in a beautiful combination".
Found in fissures in many types of rock. It is a mineraloid deposited by water in fairly low temperatures. Opal was very rare until larger deposits were discovered in the 19th century. Particularly in Australia. Hydrated Silicon Dioxide, Opals always contain water - usually between 2 and 6 per cent, but sometimes even more. If stored too dry or exposed to heat, Opals can begin to crack and flake and the play of colour will become paler. Therefore, Opal jewellery should be worn as often as possible, if it is the stone will receive the necessary humidity from the air and from the skin of its wearer.
Beware doublets (thin slivers of opal made to look like opal cabochons by adding a domed clear cap) and articial or reconstituted opal masquerading as the real thing. We see a lot of it about.