Opal is a gemstone consisting of hydrated amorphous silica with the chemical formula SiO2.nH2O. There are two basic forms of opal described by visual appearance.
Precious Opal - is opal which exhibits the phenomenon known as play-of-colour, produced by the diffraction of white light through a micro-structure of orderly arrayed silica spheres to produce changing spectral hues.
Common Opal and Potch - is opal which does not exhibit a play-of-colour. The distinction between common opal and potch is based on formation and structure. Potch is structurally similar to precious opal but has a disorderly arrangement of silica spheres. Common opal shows some degree of micro crystallinity.
Natural opal is opal which has not been treated or enhanced in any way other than by cutting and polishing. There are three types of natural opal, with varieties described by the two characteristics of body tone and transparency.
Natural Opal Type 1 - is opal presented in one piece in its natural state apart from cutting or polishing and is of substantially homogenous chemical composition.
Natural Opal Type 2 - is opal presented in one piece where the opal is naturally attached to the host rock in which it was formed and the host rock is of a different chemical composition. This opal is commonly known as boulder opal.
Natural Opal Type 3 - is opal presented in one piece where the opal is intimately diffused as infillings of pores or holes or between grains of the host rock in which it was formed. This opal is commonly known as matrix opal.
Opal is one of the 6 most precious and valuable gemstones found in the world sharing it’s prestigious beauty with diamonds, emeralds, rubies, sapphires, and pearls.
Opal is the only gemstone which can display all the colours of the rainbow ranging from violet, indigo, green, yellow, orange and red.
One of the most striking characteristics of opal is it’s “play of colour” in which the grains in the pattern of the opal display a spectral hue which can include one or more or all of the above colours that strikingly change hue as the stone is viewed or tilted on different angles. The magnificent change in colour the opal displays is caused by light waves diffracting off the tiny spheres packed within the microstructure of the gem.
The smaller spheres will diffract the colour blue, moving up the spectral hue to the largest sphere diffracting red the rarest colour found in precious opal.
Other important characteristics of opal include the base colour, patterns, brightness, making each stone unique.
The finest of opals are known to exceed the value of diamonds and emeralds per carat. Over 95% of the world’s precious opal comes from Australia.
Unlike most other gems, this amazing stone doesn't require impurities or faceting to create its phenomenal beauty. Its bursts of radiant colour are produced by diffraction of light in the same way as the rainbow. Besides, there are many differences between opal and other gems. They include an unimaginable array of alternating colours and shapes with a diversity of types. Click here About Opals for more information on the various types of Opals.