Australian opal.
Have you ever wondered how opal was formed?
The process began about 140 million years ago. At that time, much of central Australia was covered by an inland sea, abundant with silica-rich sands. This sea eventually receded and formed what is now known as the Great Artesian Basin. In Australia, precious opal is found in Cretaceous age sandstones and mudstones. These sedimentary rocks were deeply weathered and this weathering released silica into the groundwater. Small faults and joints in the rocks formed pathways for movement of the groundwater as it penetrated downwards. Impermeable barriers between the sandstone and the underlying rocks trapped the silica-carrying groundwater where it slowly hardened into a gel forming opal. It has been suggested that it took around five million years for about a centimetre of opal to develop. Opals are frequently layered and if a rare red layer is present it is at the base in the thinnest portion of the vein and indicates that gravity played a part in the arrangement of the silica spheres. Opal is found around the world, however Australia produces 95% of the world's precious opal and it is our official national gemstone. Australia is also the only part of the world where opalised animal and plant fossils have been found. In Aboriginal dreamtime stories the opal was created when the colours of the rainbow touched the earth.
Crystal opal.
The term crystal opal refers to one that is translucent or semi-translucent. Like most opal each crystal opal displays a range of colours and patterns that is unique to each stone.
Black crystal opals are transparent to semi-transparent with dark body colour and play of colour.
On a body tone chart they rate N8 or N9, but it is the brightness and pattern that determines the price for these opals
Black/dark opal.
Black opal is the most rare and highly valued form of opal. It's characterised by a dark body tone which can range from dark grey (N4) to jet black (N1). Black opals come in every colour of the rainbow, but it is the 'reds' that are more valuable. Their dark body tone makes the colours on the face of the opal appear rich and intense. The ‘darkness’ is the result of the opal having been formed on a darker quartz-like layer that enables greater refraction/reflection of the light.
Dark opal is just a few shades lighter than black opal, with a bodytone ranging between N5 and N7, and can have just as much brightness without the price tag.
White opal.
White Opal is also known as milky opal and it has a pale white appearance with bright flashes of colour and a body tone ranging from N8 or N9. White Opal is capable of displaying any combination of spectral colours such as red, blue, purple and greens. The major difference between this type of opal and black opal is that white opal tends to have a subdued play of colour since the body colour of the stone is light.
Boulder opal.
Boulder opal is a unique and beautiful opal found in Queensland, Australia. It is easily identifiable because it is a mixture of ironstone and opal either in matrix or layered. Every stone is unique, yet they are arguably the most affordable opal available.
Boulder opal is found in cracks, crevices and cavities. Silica from dissolved sand trickled down cracks and crevices in ironstone boulders and the silica precipitated out of solution. Generally, the opal is found within boulders which need to be broken open. These are found in clay layers just above the ironstone.
Doublets.
Opal doublet as the name suggests, is a thin layer of precious opal cemented onto a backing, usually obsidian, potch, host rock or basalt, then cut to make a finished stone. They were originally created from opal that was perhaps too thin to produce a solid stone.