Opal is a mineraloid, amorphous form of silica. Usually, water makes up approximately six to 10 per cent of its total weight. It is, however, possible for the water content to be as low as three per cent and as high as 21 per cent.
Deposited at fairly low temperatures, it can occur within fissures among any types of rock. It is most commonly found within sandstone, limonite, basalt, marl and rhyolite.
Opal is Australia's national gemstone, with 97 per cent of opal being produced there.
Opal diffracts light due to its internal structure. Depending on the conditions under which it was formed, it can take on a range of colours.
These colours vary from clear and white through blue, green, olive, yellow, orange and red to pink, rose pink, magenta, brown, grey, slate and black.
The most commonly found hues are white and green, with reds and blacks being the rarest colours.
The optical density of opal varies from semi transparent to opaque. When used as gemstones, the natural colours of opal are frequently enhanced by placing thin opal layers onto a darker stone base, such as basalt, for instance.
Apart from the precious gemstone varieties, other types of common opal can be found.
These include the milk opal, of a milky greenish to bluish colour and occasionally of gemstone quality; honey yellow coloured resin opal, which has a resinous lustre to it; brown or grey menilite and wood opal, caused by replacement of organic matter within wood.
Other forms are geyserite, or siliceous sinter, found around geysers and hot springs; diatomite, otherwise known as diatomaceous earth, an accumulation of diatom shells and the colourless, glass-clear hyalite, frequently called Muller's Glass.
Like other gems, the natural stones are generally quite rough and they are cut and polished for use.
Deposited at fairly low temperatures, it can occur within fissures among any types of rock. It is most commonly found within sandstone, limonite, basalt, marl and rhyolite.
Opal is Australia's national gemstone, with 97 per cent of opal being produced there.
Opal diffracts light due to its internal structure. Depending on the conditions under which it was formed, it can take on a range of colours.
These colours vary from clear and white through blue, green, olive, yellow, orange and red to pink, rose pink, magenta, brown, grey, slate and black.
The most commonly found hues are white and green, with reds and blacks being the rarest colours.
The optical density of opal varies from semi transparent to opaque. When used as gemstones, the natural colours of opal are frequently enhanced by placing thin opal layers onto a darker stone base, such as basalt, for instance.
Apart from the precious gemstone varieties, other types of common opal can be found.
These include the milk opal, of a milky greenish to bluish colour and occasionally of gemstone quality; honey yellow coloured resin opal, which has a resinous lustre to it; brown or grey menilite and wood opal, caused by replacement of organic matter within wood.
Other forms are geyserite, or siliceous sinter, found around geysers and hot springs; diatomite, otherwise known as diatomaceous earth, an accumulation of diatom shells and the colourless, glass-clear hyalite, frequently called Muller's Glass.
Like other gems, the natural stones are generally quite rough and they are cut and polished for use.
