Opal

What is Opal?
Opal is a form of silica, chemically similar to quartz, but containing water within the mineral structure. Precious opal generally contains 6-10% water and consists of small silica spheres arranged in a regular pattern.

Opal occurs in many varieties, two of which are precious opal and potch.

Colour in precious opal is caused by the regular array of silica spheres and voids diffracting white light, and breaking it up into the colours of the spectrum. The diameter and spacing of the spheres controls the colour range of an opal.

Small spheres (approx. 150-200 nm; I nm = 1 0'9 m) produce opal of blue colour only, whereas larger spheres (350 nm) produce red colour. Opal with red colour can display the entire spectrum.

Opal colours also depend on the angle of light incidence and can change or disappear when the gem is rotated.

In potch opal, the silica spheres may be absent or too small or irregularly arranged to produce colour.

potch

Potch
scanning electron
micrographs (x 40 000)

precious

Precious Opal
scanning electron
micrographs (x 40 000)

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Geology of Opal
All precious opal in South Australia occurs in rocks affected by weathering during the Tertiary Period (1.8-70 million years ago).

The weathering process broke down minerals of the country rock to produce kaolin (a clay) and soluble silica. It also created cavities in the rock by dissolving soluble minerals and fossil shells. These cavities, together with faults and fractures in the rock, provided pathways for underground water containing the soluble silica.

Periodic lowering of the watertable, possibly caused by changes in climate, carried silica-rich solutions downwards to deposit opal in the rock cavities.

Value and Presentation
Attempts have been made to establish guidelines for determining opal prices but they have been largely unsuccessful because of the gem's infinite variation in colour pattern.

The main factors influencing the price paid for opal are:

Background colour - black opal (a gem with a dark background) is more valuable than clear opal (crystal opal) which in turn is generally more valuable than white or milky opal.
Dominant fire colour - red-fire opal is generally more valuable than a predominantly green opal, which in turn is more valuable than a stone showing only blue colour.
Colour pattern - harlequin opal, where the colour occurs in patches, is generally more valuable than pinfire opal where the colour is in small specks.

There is a marked difference between the value of uncut opal and that of a cut and polished stone.

Opals may be cut and polished in a number of ways, depending on the nature and thickness of the colour band. Under the Trade Standards Act, all opal sold in South Australia must be clearly labelled to show the type of opal and how it is presented.

Solid (cabochon)
Most cutters prefer to produce the opal as a solid cut en cabochon if the gem is sufficiently thick.

Doublet
A thin veneer of opal may show enhanced colour with a dark backing. This can be achieved by cementing either black or grey silica material or a thin slice of common opal to the back of the opal with epoxy resin.

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Triplet
A slice of quartz may be used to cap the thin opal veneer to protect it from abrasion. This produces a three-tiered gemstone known as a triplet, which can often display brilliant colours. It is a cheaper method of presentation and can enhance the appearance of the opal.

Matrix Opal
Matrix comprises precious opaline silica as an infilling of pore spaces in silty claystone; it generally shows fine pinfire colour in the natural state. The colour may be enhanced by soaking the specimen in a sugar solution and then boiling in acid to deposit carbon in the available pore spaces, resulting in a dark background. Matrix opal is only found at Andamooka and is generally cut and sold en cabochon.

Synthetic Opal
Synthetic opal, such as Gilson, is opaline silica produced in the laboratory and having a similar structure to that of precious opal. The following observations can help to differentiate between natural and synthetic opal:

synthetic stones generally show brighter colours, and colour patches are often larger than in natural opal
colour grain boundaries are generally highly irregular in synthetic opal
within each colour grain in synthetic opal there are numerous sub-grains that produce a distinctive snakeskin pattern
synthetic material generally shows a more ordered array of colours since artificial material does not duplicate the intricate pattern of natural opal.

Imitation Opal
This is non-opaline material, such as coloured tinsel, set in clear plastic or epoxy resin.

Mining Opal
Miners, with a Mining Permit, can peg a claim either 50m x 5Om or 5Om x IOOm to mine for opal.

The earlier form of mining was by sinking or digging a shaft with a pick and shovel. Driving or tunnelling along the level was then carried out with picks and shovels. When traces of opal are found a handpick or screwdriver is used.

Nowadays most if not all prospecting shafts are made by using a Calweld-type drill which are used to excavate holes about one metre in diameter using an auger bucket The drills can dig to a maximum depth of about 28 to-30 metres and the opal fields are pitted with thousands of abandoned Calweld shafts.

Waste material or mullock, from the shafts and drives, was originally lifted to the surface by hand windlass, later being replaced by power winches (Yorke hoists) or automatic bucket tippers. Today truck-mounted blowers, which operate like vacuum cleaners, are more commonly used for bringing mullock to the surface.

Since the 1970's, there has been a rapid increase in the use of mining machines. Tunnelling machines with revolving cutting heads and small underground front-end loaders, called boggers, have been introduced.
There is a marked difference between the value of uncut opal and that of a cut and polished stone.

Bulldozers are employed to remove overburden and expose the level where it is shallow. Spotters follow behind watching for opal and the seam is then worked over by handpick.

Opal Mining

Noodling
Most cutters prefer to produce the opal as a solid cut en cabochon if the gem is sufficiently thick.

This is the process of searching through heaps of discarded mullock for pieces of opal missed by the miners. Many locals make a living off this method and it is popular with tourists. Permission must be obtained from the claim owner to fossick on his/her claim. The most productive heaps are those excavated by bulldozers where opal may have been crushed or overlooked by careless operators. Noodling machines, in which mullock is passed through a darkroom on a conveyor belt beneath ultra violet lights are also used, and this is another form of mining. Great care must be taken on the opal fields due to the thousands of open shafts.

Information supplied courtesy of Primary Industries & Resources, SA.