What is Opal?

 

Opal is one of the world's most beautiful and precious gemstones, predominantly found in Australia. It is one of only six types of precious gemstones found on planet earth, sharing prestigious company with diamonds, rubies, sapphires, emeralds, and pearls.

Over 95% of the world's precious opal comes from Australia, and opal is Australia's national gemstone.

While several different types of opal are present in nature, there are two main varieties - precious opal and common opal or potch. Precious opals exhibit the characteristic play-of-colour, and is comparatively rare. Opal which is dull and valueless is called common opal. Common opal occurs in abundance throughout the world. Common opal and potch do not exhibit a play of colour. When common opal is found in association with precious opal, it is known as potch. About 95% of all opal mined from the opal fields is common or potch, that is opal that is basically one coloured, i.e. white, grey, black, and is only suitable for backings for doublets or triplets. Of the 5% that has some colour, about 95% is only of mediocre grade - therefore only approximately 0.25 per cent has any real value at all.

Scientifically, opal is known as SiO2·nH2O - an amorphous mineraloid comprised of hydrated silicon dioxide. (A mix of silica and water). The opal's appearance can range from colourless, white, and light grey through to dark grey and black. However the truly unique thing about opal is that it displays all the colours of the spectrum in a "play of colour", resulting from the interference and diffraction of light passing through tiny silica spheres in the microstructure of opal. This means the bright rainbow colours of opal, as they appear to the human eye, will move and change dramatically depending on the angle at which the stone is viewed.

A definition of opal : "An amorphous non-crystalline gem mineral solidified from gelatinous or liquid silica deposited in cracks and cavities left by decaying vegetation, wood, crustaceans and bones. Very valuable in its 'black' forms and containing a reasonable content of water. Chemical symbol: SiO2 plus H2 O. The water content in opals can be as high as 10%. Refractive Index of 1.38 - 1.60 and a hardness of between 5.5 to 6.5 on Mohs' scale."

Precious opal is defined as opaline silica with a play of colour. The term, play of colour was created solely to describe opal's beautiful shifting of spectral hues. Australian opal is referred to as "sedimentary opal" because it is hosted predominantly by sedimentary rocks of the Mesozoic Great Artesian Basin. Australian precious opals usually contain around 5-6% water and consists of small silica spheres arranged in a regular pattern. Its hardness of 5.5 to 6.5 on Moh's scale puts it about half way down the range (roughly the same hardness as glass). It has a specific gravity of 1.9 to 2.3 depending on the amount of water present.

 

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