Cutting and polishing of Opal
When one is cutting opal, the first thing to do is to have a close look at the rough material. If it is full of colour or are there only bars of colour, that means the colour goes in stripes through the potch. If so, are these bars straight or uneven? If they are not straight, the cutting can be difficult, time consuming or even impossible. The quality of the material is important as well, if it is very expensive the wastage must be kept to a minimum. But even a good cutter can lose up to 30% of material when cutting.
Opal is seldom cut with facets because facets bring light into the stone, which is not desirable as the light has to enter from the front for a good refraction. There are only few stones which show good colour when the light shines in from the back. (Contra Luz Opal from Mexico).
The first step usually is to cut the opal to suitable pieces on a diamond saw which turns with 5000 rpm. Then it is taken to a grinder where the stone is roughly shaped. Afterwards the stone is put on a dopstick, usually a stick of wood, and held there with wax or glue.
Now the work on the grinder starts. The grid of the grinding wheels ranges from 180 to 1200. Opal from Coober Pedy is usually ground on 180 or 360 wheels with 2000 to 2800 rpm. With turning movements the opal is cut to the desired shape, cabochon or freeshape.
After grinding, the surface is not yet smooth. Therefore the stone is taken to the sander, a turning disc (500-750 rpm).The opal is cut on the flat side of the disc on sanding paper (grid 180-800). The sanding paper is stuck onto 5mm of rubber. This gives in to some pressure and with rolling movements of the stone, a smooth surface is created.
The next step is to polish the stone on a felt wheel. Cerium oxide or lapsa are used as polishing agent together with water. Now the surface is finished. The stone is taken from the dopstick and turned around to cut the back. The back should be only slightly curved with a setting edge around.
Jewellery and Retail
When it comes to jewellery, there are two forms of opal. Either they have a calibrated size or it is a freeshaped stone. For the stones with calibrated size a variety of settings are available from jewellery suppliers. This makes it easy for the jeweller to set a stone, and it is (or should be) less expensive for the customer. Most Triplets are calibrated, many doublets too and the lower or medium quality solid opal as well. The disadvantage is, that these settings are not too special. It does not mean that they are cheap or cheap looking. But it is mass-production and not an individual creation which good opal deserves.
The freeshape-stones are often not easy to set. They are mainly good quality opal, where a calibrated cut would have been a loss of weight. They may be cabochon shaped, but can just as well be shaped rather uneven. Opalised fossils like shells or pipes are very special to set. Just like carvings (done with a dentist drill) they demand a skilled jeweller who likes the artistic aspect of his trade.
It is said, that the price of opal from rough to cut increases about 1:5, which is an average. A buyer of rough opal may be lucky and get opal for a low price and after cutting it turns out very well. But it is not always like this. He might be very unfortunate and the opal cracks during the cutting or afterwards. Sometimes the buyer does not know from which field the opal comes, therefore it is a risky business as opal from some fields tends to crack. There can be high profits and high losses.
Opal should be stored out of extreme heat or cold to avoid cracks. A bowl of water near the opal will keep the humidity of the air up so the opal can not dry out. To store opal in water can avoid cracks, but good opal is usually stable and will rarely crack. Rough opal is often sold in glasses or jars with water, simply because the wet surface looks like polished and the colour shows better. Personally, I prefer to buy rough opal that has been out of the ground and kept away from water for at least a few weeks, so either it cracked during this time or is stable.
What makes the price of opal?
It is very difficult to estimate the value of opal because every stone is individual. If somebody likes a stone enough they are willing to pay the price. The most extreme example are probably stones with so called Chinese-writing pattern. In some cases a character with a meaning appears and who could say how much it is worth to a certain person? The value can be very personal, but there are a few factors to determine the value to a certain degree.Ĺ
Here is a list what is important: (Andamooka matrix not considered)
· Background: Black (rare and expensive), Crystal, Grey, White, Jelly, Boulder...
· Brightness: the less light a stone needs to show colour, the better.
· Pattern: Harlequin, Rolling flash, Pinfire...
· The interplay of colours (dominant red is considered best)
· A multidirectional fire, e.g. when the stone is turned it always shows colour.
· And of course the quality of the cut and polish.
- Paul B. Downing: Opal, Identification and Value, Tallahassee FL, U.S.A., 1992
- Barry O’Leary: A field guide to Australian Opal, Victoria, Australia, 1986
- Andrew Cody: Australian Precious Opal, Melbourne, Australia, 1991