Classed as a hydrated amorphous or non-crystalline form of silica, opals have a unusual internal structure which means they diffract light. Precious opal comes in many colours depending on how and where it was originally formed. One piece may contain a number of different colours with the rarity of the combination and brilliance of the light determining the value. Unlike precious opal potch or common opal doesn't show flashes of colours. The most valuable and spectacular of the Australian opals is the black opal and red on black is the most valuable of all.
For some reason opals have a reputation as unlucky but that doesn't stop their popularity for use in jewellery. The best pieces are shaped into cabochons but even pieces too thin to cut and shape can be used with a backing to make beautiful pieces of jewellery.
Opals don't only form from deposits in rocks although this is the most common. They are also found in the form of fossilised wood and tree roots, even as opalised dinosaur bones and marine creatures.
The major opal fields in Australia are around Coober Pedy - where many dwellings are underground to deal with the extreme heat - and Andamooka in South Australia and Lightning Ridge in New South Wales although there are some smaller fields in south west Queensland.