Opal falls into one of two categories, precious opal or common opal. Precious opals boast an iridescence that's known as "play of colour" which has been highly prized since Roman times whilst common opal lacks play of colour but can be found in many different colours. The play of colour in precious opal is caused by the reflection and scattering of light from the minute uniformly sized and closely packed silica spheres that make up this stone. Around ninety five per cent of the world's opals come from the outback deserts of Australia and the remaining five per cent are mined in Mexico, Brazil, the U.S states of Idaho and Nevada and more recently, some have also been found in Ethiopia and Mali. The opal in the photograph at the top of this page is a 54.7 carat Ethiopian opal that's currently on display in the Smithsonian National Museum of Natural History in Washington D.C.
Opal was found by Australian gold prospectors in 1863 but the stone is believed to have been mined much earlier by the Aztecs in South and Central America.
As with many gemstones there are countless legends and myths which surround the opal. One story comes from the Australian Aborigines who believed that God came down to earth on a rainbow in order to bring the message of peace to all humans. At the very spot where his foot touched the ground the stone came alive and sparkled with all the colours of the rainbow.
The origins of the name opal possibly came from the Sanskrit word 'upala' meaning 'valuable stone' and this in turn was probably the root for the Greek term 'opallios' meaning 'colour change'. The Ancient Greeks believed it could give the wearer the power of foresight whilst the Romans revered it as the symbol of hope and purity and believed it could protect from disease. Eastern people regarded opals as symbols of truth and the ancient Arabs believed it came from heaven and acquired the play of colour from flashes of lightning. During the Middle Ages it was thought to be beneficial for eyesight and some even believed it could render the wearer invisible hence it was often carried by thieves. The French Emperor Napoleon gave his wife Josephine a magnificent opal called 'The Burning of Troy' because of its variation of colour.
A superstition brought about by the novel Ann of Geierstein written in 1829 by Walter Scott damaged the popularity of the opal because it was described as being an unlucky stone but its reputation was gradually restored by public figures such as Queen Victoria and the French actress Sarah Bernhardt.
The common opal which is not as well known or as popular as the precious variety can be found in many different colours but the most highly sought after stones are pink and green. It's almost always opaque and the only reason it's called "common" opal is because it can be found in many countries around the world. Like the precious opal the common variety is almost always cut and polished as a cabochon.
Boulder opal is another variety of opal which forms in thin veins within ironstone boulders. These are fine grained heavy and compact sedimentary rocks whose main components are the oxide of iron, clay and/or sand. Freshly broken ironstone is usually grey however the brown external appearance is due to the oxidation of its surface. Ironstone can also be found in a red and black banded form and when this mineral is polished as a gemstone, it's better known as Tiger Iron.
In crystal healing opals are said to help with depression and to help the wearer to find true love. They are also said to enhance the positive characteristics for people born under the zodiac sign of cancer.
Opal is a fragile stone which needs to be well looked after. It grades 5½ to 6½ on Mohs scale of mineral hardness and along with tourmaline, is the birthstone for the month of October.