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.
Here are some great examples:
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 dak 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.
Most cutters prefer to produce the opal as a solid cut en cabochon if the gem is sufficiently thick.
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.
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.
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.