Opal is a variety of quartz often called the ‘paradoxical gemstone’. This is because it is particularly difficult to categorise – there are many varieties, some of which display many colours, others of which are colourless, some of which are particularly bright, others of which have only a dull sheen, some of which are black and others of which are white. Classical opals contain flashes of colour and an iridescent colour display which seems to glow from the centre of the stone and across its surface, often colloquially described as ‘fire’, and known as opalescence precisely because of its occurrence in this stone. Similar phenomena can be observed in labradorite and moonstone.

The majority of opals are ‘light opal’ – white or crystal opal. Only 8% of opals are black. Other varieties include fire opal, a translucent to semi-opaque flame-coloured stone that demonstrates substantial pleochroism and comes from Mexico. Peruvian opal (also known as blue opal), another variety, is a semi-opaque to opaque blue-green stone which, as the name suggests, originates in Peru. Peruvian opals do not display pleochroism.

Because of the differing percentage of water, opals can easily become brittle. They usually contain between 2% and 6% water, and sometimes even more. If they are stored in too dry or warm a place over a substantial period of time, they therefore begin to show cracks and lose their radiance. To prevent this and retain their beauty, opal jewellery should be worn as often as possible to ensure that the gemstone receives the needed humidity from the air and the skin of its wearer.

The name ‘opal’ is said to have evolved from the Greek word opallios, meaning ‘to see a change of colour’. The Greek word was apparently itself a modification of the ancient Indian Sanskrit name for opal, upala, meaning ‘precious stone’. The ancient etymology of the word demonstrates for how long and to what extent opal has been prized as a gemstone.

Historically, there have been innumerable beliefs associated with the wearing of opal. The early Greeks believed that opals gave their owners the powers of foresight and prophecy, while the Romans treasured it as a symbol of hope and purity. It has long been regarded as sacred in the East, where Arabs believed that it fell from heaven.

In the Victorian era, a superstition grew that deemed it bad luck to wear opals unless it was one’s birthstone. This belief has largely diminished nowadays, perhaps due to the particularly alluring beauty of opals in fine jewellery.

The Aborigines have a particularly beautiful legend surrounding the origin of opals. It is said that the creator came down to Earth on a rainbow, in order to bring a message of peace to all mankind. As his feet touched the ground, the stones around him sprang to life and began to sparkle in all he colours of his rainbow – this was the birth of opals.

Opals were mined by the Romans in Cervenica, previously in Czechoslovakia, although we have now lost track of all the recent changes in Balkan geography.

Today, Australia produces around 97% of the world’s black and white opals. Due to its abundance there, the opal is the official gemstone of South Australia and the Commonwealth of Australia. The Australians women's national basketball team is also nicknamed The Opals.

A deposit of opals discovered in 1930 is still being mined in Brazil, and some black opal mined from volcanic ash deposits originates in Honduras. Opals are also found in Nevada, where there is a mine at which the public have the opportunity to mine the gems for themselves.