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 the total mined.
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During the Cretaceous period (65-140 million years ago) deserts were an inland sea teeming with life. The sea gradually receded laying the foundation of silica that would form the opal of today. During the mid-tertiary period, changes in the earth’s climate caused quantities of soluble silica to be released from the 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 the opal is from the uniform alignment of the silica particles. The size of the particles determines the 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 the total mined.
Black Opal:
Black opal is the 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 the world.
Boulder Opal:
Boulder opal, found in the 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, the stone is cut leaving the natural host rock as backing. The opal occurs as either a solid piece of colour on top of the ironstone or showing as flashing flecks of colour throughout the 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 the background colour either white or light blue.
Crystal Opal:
As the name suggests, crystal opal has a brilliant, crystal appearance, with a translucent or transparent background. When viewed on a dark surface the colours spring to life.
Picture Opals:
Picture opals are so called because their pattern forms an image of an object or person. A good imagination is sometimes required to visualise the object/person.
Fire Opal:
The term “Fire opal” is commonly used to describe the 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.
Opalised Fossils:
Buried in the layers where opal mining occurs are the remains of a world over a hundred million years old, brought to the surface by miners searching for gemstones. These fossils include the 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 the organic material.
Common Opal:
Common opal is classified as non-gem quality opal, mostly opaque and showing no play of colour.
Matrix Opal:
Matrix opal is when opal is infused in the rock in which it was formed. Veins of precious opal permeate sandstone or ironstone filling holes and crevices of the host rock.
Synthetic Opal:
Synthetic opal as the name suggests, is produced in the laboratory and has a similar structure to that of precious opal.
The following observations can be made to differentiate between natural and synthetic opal:
a. Synthetic stones show brighter colours and larger colour patches;
b. In synthetic opal, colour grain boundaries are highly irregular;
c. Synthetic opal has a distinctive snakeskin patter;
d. The material shows a more ordered array of colour because the intricate patterns of natural opal cannot be duplicated.
Rough Opal Parcels:
Rough opal parcels are sorted into three grades: Top, Middle and Low.
Each Top parcel has a King Stone (the best stone in the parcel). Colour is the primary criteria for grading.
You can buy rough opal in several different conditions:
a. Mine Run – Directly from the miner himself. These stones have not yet been cut or graded.
b. Off Cuts – The miner has removed all marketable opal and you are sold what is left. With off cuts, you can usually tell what you are going to be able to cut.
c. Rubs – The opal stones have been cut and ground into basic shapes, after having removed most of the rubbish.
Andomooke Matrix:
A more porous opal found in the mining fields. Out of the ground it is quite pale, but after treating it with a carbon dye process it looks like a real, black opal.
Yowah Nut:
This rare and unusual opal, up to 5 cm across, looks like a round or oval nut and is an ironstone formation.
Opal forms within the nut either as a solid kernel or more commonly, in concentric layers infused with the host rock. The nut has to be split open before the contents are revealed and it may be split so that mirror images are produced.
Opal Doublets:
Opal doublets, as the name suggest, is a thin layer of precious opal cemented onto a backing, usually a piece of black common opal or ironstone. Produced because the opal is too thin to produce a solid stone, doublets are a cheap and inexpensive way to buy a colourful piece of precious opal without the cost involved of a solid.
Triplets:
Similar to a doublet, but has three pieces sandwiched together, with a thin piece of precious opal cemented to a backing and a clear quartz cap placed on top to protect and bring out the colour of the opal.
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