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random shapes
opal doublet
high quality triplets
triplet brooch
boulder opal
yowah nuts
PRESENTATION OF OPAL

Treatment

The stones are marketed in a variety of forms and shapes designed to make the most of the beauty of sample of raw opal. Thus, some raw material may yield a solid opal gemstone, while other samples may prepared as natural stones or man-made composite.

These are described in the following sections:

Solid Opals
Doublets
Triplets
Boulder opal
Treated opal matrix

Solid Opals

When the opal is thick enough, and especially if the quality is high, opal is normally cut into cabochons, usually of standard sizes. With very good quality stone, random shapes may be used to preserve as much of the colour as possible. Solid opals command, of course, much higher prices than triplets and other prepared forms of opal.

Doublets

 

Doublets are made from thinner pieces of opal cemented onto a, preferably dark, background material. Dark coloured potch is the preferred backing material as it has the same physical characteristics as the gem material. Sometimes, however, dark glass is used as a backing. The opal in a doublet may be cut from a solid piece of stone, but may also be cut from thin veins from which it would be difficult or impossible to cut a solid stone. The layer of opal may be a millimetre or more thick, but the stones are somewhat fragile and must be treated with care. However, doublets are less common nowadays, having been largely replaced by triplets.

Triplets

Triplets are made in the same way as doublets, but a third layer of transparent material is cemented onto the top of the stone. This cover is in the form of a cabochon, and serves to protect the thin layer of precious opal. The cover piece was formerly cut from quartz, making a stone which was quite resistant to wear. nowadays the cover for a triplet is more commonly made from glass, or in cheaper stones, plastic. The difference is not evident, except that the glass and plastic, being softer than quartz, will be less resistant to wear.

The opal layer itself is generally thinner than with a doublet. The quality varies greatly, but a high quality triplet, when well mounted in jewellery, can look very like a good solid opal. Triplets are usually made in the form of cabochons, and come in standard sizes. Good quality pieces are sometimes cut into random shapes to preserve as much of the colour as possible. Cutting opal as triplets may also enhance the colour of poorer quality opal, as the thin layer is less prone to scatter white light, and hence looks less milky.

Boulder Opal

 

Much of the opal recovered from the Queensland fields occurs in the form of boulder opal. This opal is found in ironstone (goethite) concretions up to 30 cm or more in size. The opal may occur as thin veinlets or impregnations in the ironstone, but the best material occurs as cores in the boulders and can be quite exceptional, approaching Lighting Ridge black opal in quality.

When the opal core is thick enough it can be cut as a normal solid cabochon. Frequently however, the opal layer is thinner, so that it is cut in such a manner as to leave a backing of ironstone. This forms a natural doublet, adding strength to the stone, and provides a dark background which enhances the colour.

Lower quality material in which the opal forms a network of thin veinlets, or otherwise impregnates the ironstone, may be cut as large cabochons which consist largely of goethite. This results in attractive brown stones brought to life with patches and veinlets of opal colour. This is often termed 'boulder opal matrix'.

In the region of the small town of Yowah in south western Queensland, smaller concretions, a few cm or so in size, are mined. They are called 'Yowah nuts'.

Treated Opal Matrix

Certain types of opal, found especially at Andamooka, which show little colour when mined, have a fine porosity which can be used to enhance the colour. The stone is soaked in an aqueous solution of lactose (sugar of milk), and then treated with warm sulphuric acid. This breaks down the lactose, and the carbon deposited in the pores gives a black background appearance to the opal. Because there is no longer any white light scattering, the colours inherent in the stone are enhanced. The quality varies greatly, but the better quality material is attractive, and probably much underpriced.

Occasionally the results can be remarkable. The late Harold Hodges, a well known identity in Lightning Ridge in the 1960's, possessed a fine black opal of some 30 carats, for which he was offered the then equivalent of $20,000 by a dealer. Harold told the dealer to look again at the stone, carefully. According to Harold, the dealer went white; the opal was a treated matrix of such quality that it could easily be mistaken for a high quality natural black opal!

Those with little knowledge are also sometimes taken in by less than scrupulous dealers selling treated matrix. The writer was once asked by the police to examine some opals which had been bought by an American tourist for $15,000. There were five stones amounting to somewhere about 120 carats. They were, however, good quality treated matrix, worth about $500 at that time. It was suggested that there was a case for fraud against the dealer. However, the opals were accompanied by an elaborate valuation certificate, issued by a well known opal expert, saying that the stones were genuine 'treated matrix', upon which a value of $205 a carat was placed. The stones were correctly described, so there was, in law, no case. Caveat emptor! It is interesting to note that artificial darkening of opal is referred to in an early edition of Chamber's Encyclopedia (Volume 7, 1881):

"What lapidaries call Prime d'opal is clay-porphyry, or other stone containing small grains of opal. It is cut into slabs, and made into boxes and other ornamental articles; the stone which contains the opal being artificially blackened by boiling in oil, and afterwards exposing to moderate heat."