The world’s highest quality precious opal is arguably found in Australia, which is home to larger opal fields than those in the rest of the world combined. But not all opal is precious, and depending on its structure and composition it may take on many qualities, from beautiful iridescence to simple translucence.
Precious opal exhibits a brilliant play of colours that change with the viewing angle – a property known as iridescence. This type of opal has an internal three-dimensional structure arising from the regular packing of sub-microscopic spheres of silica. This structure diffracts light causing the spectacular iridescence.
While the body colour of most precious opal is white, it can also be black, transparent or red-orange. The latter is known as fire opal and its colouring is caused by the presence of small iron oxide impurities, or rust.
Common opal is translucent and does not show any play of colours, but the body colour differs depending on the impurities that are present. Opal has a chemical composition of SiO2.nH2O and is a mineraloid; unlike minerals, mineraloids have no ordered atomic structure.
Opals are comprised of hydrated silica and typically contain 6-10% water. They form during the acidic weathering of rocks, or precipitate from water-rich fluids containing silica which fill cracks in rocks.