Opal is the national gemstone of Australia. It is the October birthstone and the traditional wedding gift for the 14th anniversary.
Australian opal occurs as deposits within Cretaceous sedimentary rocks and accounts for over 90% of precious opal found in the world.
Opal is a mineraloid gemstone consisting of millions of tiny amorphous silica spheres which diffract light, taking on different colours from various angles. This happens when light enters the opal and is split on the interface of small voids which have been formed between the spheres.
The size of the voids is controlled by the diameter of the spheres. The angle at which the light is split is critical and determines the colours that are exhibited. Larger spheres result in diffraction at the red end of the spectrum, and smaller spheres diffract at the blue/green end. The more even the spheres, the greater the areas of single colour.
Boulder opal is a type of opal that forms within the cavities of ironstone boulders in both vertical and horizontal cracks. It is cut with the ironstone remaining on the back and is classified as solid opal. It occurs over a wide area of Western Queensland from North of Winton to south of Quilpie.
It is the second most valuable Australian opal after black opal. When looked at from the side you can always tell a genuine solid boulder opal by the natural uneven line where the opal filled the void in the boulder. Boulder opal is finished in many different shapes rather than the traditional oval of many other stones and as such is more unique and interesting. It is famous for its brightness and variety of colours with all the shades of other types of opal and can be dark or light body tone.
Koroit opal It is formed in a similar way to boulder opal except that instead of filling voids in ironstone boulders the opal is scattered throughout the boulder in interesting patterns and lines or as a nodule of colour in the centre of the boulder which is known as Koroit or Yowah-nut. Koroit opal is found near Cunnumulla in south western Queensland.
Black opals are solid opals with dark body tone and varied play of colour. These opals are the most highly valued and are found in Lightning Ridge in New South Wales. A black opal?s base is generally black, dark blue, dark green, or a deep gray and they are usually found in what are known as ?nobbies? (small, slightly flattened out concretions) or as seam opal. It can feature all the colours of the rainbow but red on black background is most valuable.
White opal has a light or white body tone and occurs predominantly in Coober Pedy, Andamooka and Mintabie in South Australia. It is found in seams and sometimes replaces fossils like shells and bones. It features a myriad of colours which are more muted due to the light body tone. It is often referred to as milky opal.
Crystal opal is a term used for a transparent to translucent opal which has a play of colour within the stone. Crystal opals are often faceted to produce gems that have internal play of colour and fire exiting from the facets. They are found in the Coober Pedy and Lightning Ridge opal fields.