During Cretaceous period (65-140 million years ago) deserts were an inland sea teeming with life. The sea gradually receded laying foundation of silica that would form opal of today. During mid-tertiary period, changes in earth’s climate caused quantities of soluble silica to be released from sediment, finding its way down through cracks and faults, eventually hardening over time. This hardening continued to form common and precious opal.
The vivid array of colour in opal is from uniform alignment of silica particles. The size of particles determines colour as light refracts through it.
Unlike many other gemstones, opal doesn’t occur in lengthy veins or concentrations. Instead, small clusters may be spread over a vast area and divided into precious or gem quality and common. Opal is found in many varieties, but precious opal represents only a small percentage of total mined.
Black opal is rarest and most valuable of all opals and is generally found as a bar of various colours in a dark body. In addition, there is also Semi black and black Crystal.
Black opals from Lightning Ridge, Mintabie and Andamooka in Australia account for 99.9% of all opals in world.
Boulder opal, found in fields of Queensland, is classified as a solid opal and occurs as thin veins of precious opal in cracks and cavities of ironstone. During processing, stone is cut leaving natural host rock as backing. The opal occurs as either a solid piece of colour on top of ironstone or showing as flashing flecks of colour throughout stone.
Light Opal (White or Milky):
Light opal can be found in Coober Pedy, Andamooka and Mintabie in South Australia. A full range of colours can be found, with background colour either white or light blue.
As name suggests, crystal opal has a brilliant, crystal appearance, with a translucent or transparent background. When viewed on a dark surface colours spring to life.
Picture opals are so called because their pattern forms an image of an object or person. A good imagination is sometimes required to visualise object/person.
The term “Fire opal” is commonly used to describe clear orange crystal opal that comes from Mexico, some of which have a play of colour, although many have just an orange or reddish base with no play of colour.
Buried in layers where opal mining occurs are remains of a world over a hundred million years old, brought to surface by miners searching for gemstones. These fossils include remains of dinosaurs, shells, mammals, plants and fish. Some of these fossils have been opalised, a process in which silica-laden waters have gradually replaced organic material.
Common opal is classified as non-gem quality opal, mostly opaque and showing no play of colour.