Opal is made up of close packed aggregates of silica spheres, and with a water content between 3-10%. In precious opal the arrangement of spheres is in orderly layers, and light passing through the spheres is diffracted at the void and layer interface to produce the vivid play of colour associated with opal. Larger silica pheres are associated with more sought after colours, such as red.

It has hardness ranging between 5-6.5, is brittle with a conchoidal fracture and some light varieties fluoresce white or yellow under long and short wave ultraviolet light. This property is used on South Australian fields such as Coober Pedy to noodle opal from tailings, which are run on conveyor belts through darkened sheds, past UV lighting.

A large percentage of the worlds stable commercial opal comes from Australia, and there are three distinctive regions where opal comes from. These are Lightning Ridge (the home of black opal) the South Australian fields Coober Pedy, Mintabie, Lambina and Andamooka (the main sources of light base opal) and the Queensland fields - too many to list - where boulder opal originates. All these fields loosely border and ancient inland sea (click map) and as a result opalised marine fossils are found.

Having spoken to many jewellers and opal cutters at Tucson, there seems to be widespread confusion about what constitutes a black opal versus a light or crystal opal, and this is not helped when conflicting information is given out by separate dealers in quick succession. Some of the confusion seems to arise from the difference between Austalian and US gemmological terminology.

In Australia, a black opal clearly shows signs of looking black - if it doesn't (either the background potch or the face background) it is not a black opal. The blackness is significant because this is what helps throw the colour out and makes a stone look bright.

Cystal opal in Australia is considered to be opal that is transparent or semi transparent. If it is clear like glass with flecks of colour it is often called jelly opal, though sometimes this term is used to describe transparent crystal opal that has body colour but not real fire or pattern (looking what what Australians call jelly or Americans call jello). In the US it seems that crystal opal is described as anything you can read writing through.

In the US it is seems to be not unusual for anything that is not crystal or stark white opal to be called black opal. particularly if it is grey opal.

In the South Australian opal fields (Coober Pedy, Mintabie, Lambina) where most opal is light base anything that is not crystal or bleach white is called grey. At Lightning Ridge, the home of black opal, these same stones would be called white or light base. At LR the term grey is used to describe material that has a mid to dark grey potch backing or grey background in the face.

Black opal is generally more sought after and more valuable than light opal. This is because it often appears much brighter (particularly under fluorescent light where light opal fades) with more saturated colours and more interesting broad patterns.

In Australia the major opal producing fields for black opal are Lightning Ridge in New South Wales and to a much lesser degree Mintabie in South Australia (Please note it is not spelt Mintubie!) The most sought after black opal is red or multicolour on black, which is very difficult to find. Such stones are always cut on the fields and sell immediately.

Boulder opal is opal that has formed in a brown ironstone nodule, and is distinctive because it typically presents as a thin layer of opal over a dark brown base when cut. The opal can be quite superb. However it was quite interesting to see many doublets made from South Australian light opal with ironstone backings being sold as boulder opal at Tucson this year!

There are a number of synthetic and imitation opal products in the market place. Slocum stone was an early imitation product, in which fine coloured tinsel like threads were embedded into glass. More recently synthetic opal was developed by Pierre Gilson. This is chemically similar to natural opal, and but on the upper end of the hardness scale, and has no water content (whereas most natural opal has 6-8% water). A man made Japanese opal product with a plastic base has become wide spread over recent years, and most recently Russian synthetic opal has found its way onto the market place. Details of how this Russian product is produced can be found on the Australian Gemmologist website at http://www.austgem.gil.com.au under "Important Papers".

Lightning Ridge opal is usually found as nobbies - small blocks, pillows, spheres or hat shaped stones ranging from around 1-5 cm across. The stones usually have light grey appearance when found due to a thin outer layer of grey potch. When stones are clipped they reveal black potch inside along with any colour bar. The difference between the outside appearance of a nobby and its inner fire and black colour when snipped or ground can be quite dramatic. Consequently it is not uncommon for good stones to be missed (and thrown on the driveway as scraps)!

Opal from Lightning Ridge is often considered to be the best and brightest in the world.

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