>>click to enlarge<<
cracked potch opal
network of fine cracks
veins of satin spar
idiomorphic
quartz crystals
PRESENTATION OF OPAL

Imperfections

There are a number of factors which may affect the value of opal, just as certain imperfections such as inclusions affects the value of other types of gemstones. In fact, imperfections in many gemstones are the key to distinguishing them from synthetic and artificial stones.

Sometimes natural stones are treated in some way to enhance their appearance. Sapphires, for example, are frequently heat treated to change the colour, or to clarify stones which contain 'silk'. Cracks in diamonds can be healed by impregnation with a glass of the same refractive index as the diamond; the result is, in this case, difficult to detect. Cracks in emeralds are sometimes temporarily hidden by treatment with green oils, so let the buyer in some countries beware! In fact, a considerable proportion of gemstones are nowadays treated in some way. In many cases, as with sapphires, the quality of the stone is improved permanently.

These are described in the following sections:

Cracking & Crazing
Dead Spots
Gypsum & Other Inclusions

Cracking & Crazing

A problem occurring occasionally with opals is that of cracking or crazing. This may appear as a very fine network of tiny craks throughout the stone, or as a few longer cracks which may or may not traverse the stone. Sometimes, a crack can be seen penetrating a stone, but gradually disappearing towards the centre.

The cause of cracking is not entirely clear, but is probably in part due to loss of water. Most opal remains stable indefinitely; the writer has opal from Andamooka which is still in its pristine condition after more than 60 years.

Cracking has been known to occur soon after bringing the stone to the surface, or may only occur months or years afterwards. An experienced miner at Coober Pedy claimed that he had found opal at a depth of one metre, but the stone had cracked after exposure to surface conditions, although this appears to be a rare phenomenon. On the other hand, he had found and brought up opal from moist environments at a depth of 25 metres without any subsequent problems.

Certain areas have a reputation for producing suspect material. Occasional areas around Coober Pedy have produced some cracky material, and that from the Rainbow Ridge field in Nevada, U.S.A., while often being some of the most beautiful opal, is known for its predilection to crack. With few exceptions, opal from Australian fields is known to be quite stable.

Opal from igneous rocks, such as that from Tintenbar in New South Wales, or from volcanic areas in Japan and Indonesia, for example, shows a marked tendency to crack, probably because of its higher temperature of formation in the first instance.

Cracking, and especially fine crazing, does not necessarily render the stone physically unstable. Fine crazing in particular, with its network of fine cracks, means that the stone is unlikely to chip or break. Some such crazing may be so fine that it is not evident to the naked eye. The fine cracks merely enhance any milkiness in the opal by assisting the scattering of white incident light. I have seen an old necklace of milky opal beads which appeared perfect to the naked eye, but, on careful examination with a lens, the beads were seen to be crazed on a very fine scale.

The cracks themselves are usually very thin. This is shown by the way a crack can penetrate only part of the way into a stone, and then disappear. Even in a scanning electron microscope it is sometimes difficult to see the crack at the surface of a polished stone. However, part of the reason in this case could be the possibility that the polishing process has smeared a thin layer of silica over a pre-existing crack.

In some cases a crack may be open enough to allow the entry of water, which can render the crack virtually invisible; on drying, the crack reappears. This phenomenon has long been known in the common opal called 'hydrophane'; it is a milky colour, but becomes transparent on immersion in water. Such a phenomenon is illustrated by a wood opal from Lake Eyre in South Australia. It consists of milky and clear bands; on immersion water, the milky bands rapidly become transparent. Scanning electron microscopy showed that the milky material contained a fine network of cracks through which the water could penetrate the opal.

It is questionable whether a good method of healing cracks in opal exists. There are claims that there are methods to hide the cracks, but this may only be a temporary effect. The cracks themselves are mostly very thin, and it is likely than any successful healing technique would need to be carried out on the rough if already cracked, or on a cut stone if subsequently cracked.

In general, the best solution to avoid this occasional problem is to buy from a reputable dealer!

Dead Spots

Opal is very variable in colour quality, and, at times, small spots of little colour can occur in stones of otherwise good colour. Little can be done about such small areas, although the skilled cutter can keep these spots to a minimum in the final stone by careful cutting to maximise the colour areas.

Gypsum & Other Inclusions

Gypsum occurs on all of the opal fields, and is particularly abundant at Coober Pedy, where it often forms attractive veins of the so-called 'satin spar'. Occasionally, tiny patches of gypsum, recognisable in most cases by its fibrous crystals, may be found as inclusions in the precious opal. This problem, as with dead spots, may be alleviated to some extent by the skilled cutter.

Other types of material, such as iron oxide, can also occur, but are less common. In at least one case idiomorphic quartz crystals were observed; these appear to have grown in the opal, presumably while it was still in the condition of a soft gel.