Varieties of precious Opal
Natural Opal is divided into the categories of black, dark, light, boulder and matrix opal. The classification of black, dark and light is determined by the body tone i.e. the degree of darkness in the background.

Black Opal is the most valuable and comes mainly from Lightning Ridge. High quality stones are very rare; this type is easily distinguished by the blackness of the base or background ‘body tone’.

Black Crystal Opal is mainly mined at Lightning Ridge. These black opals show a degree of transparency, the colours are often brilliant and can appear to come from within the depth of the gemstone. Good black crystal opals are very rare.

Dark Opal comes from all fields and is desirable because the colours are generally more brilliant due to the dark background ‘highlighting’ their colours. This type of opal mainly comes from Mintabie (SA) and Lightning Ridge (NSW).

Light Opal is usually found at all opal fields but the bulk of the material has come from the South Australian field of Mintabie, Coober Pedie and Andamooka, although the first material was mined at White Cliffs (1887). This opal has a light body tone.

Crystal Opal embraces opal which is transparent of very translucent and in the better qualities shows a distinct and very bright play-of-colour. This type of opal is found in most Australian opal fields.

Boulder Opal is composed of opal naturally occurring on its host rock, and is mined predominantly in Queensland. It is easily identified because, when cut, the host rock brown ironstone) is left on the back of the opal. Boulder opal may be light, dark or black. In the last 20 years this type of opal has become extremely popular as it can display the same darkness and brilliance as the high quality black opal.

Matrix Opal has two common types in Australia:

• Boulder Matrix Opal is usually found in Queensland. The opal is intimately diffused with the host rock, (usually ironstone). The host rock is quite obvious in the presentation face of the stone.
• Matrix Opal from Andamooka has opal intimately diffused throughout the host rock, sandstone which is often porous and can be treated, turning the material dark resembling black opal.

There are three main forms of Composite Opal

• Solid Stones, as the name indicated, are cut from a single piece of solid opal.
• Doublets consist of a slice of opal with a backing of potch to make the best use of a thin piece of opal or to provide a darker background to enhance the colours.
• Triplets consist of a thin slice of opal sandwiched between a backing of potch and a protective cap of quartz or glass. This allows a much thinner slice of opal to be used and also provides protection for the opal, which is somewhat softer than other gemstones.

Factors determining the value of all types of Opals

• The brilliance of colours is of paramount importance — the brightness of an opal is directly related to the price.
• The patterns of colours when combined with brilliance may increase value many times.
• Colours: The number of different colours affects price, although this rule should be used with great care. An opal with all the colours of the spectrum is not always more valuable than one displaying red/blue or green/blue alone, because the major determination of value is brilliance and pattern.
• Shape: usually cut en cabochon; a stone with a domed surface will be more valuable than a similar stone with a flat surface. Opal which is too thin could fracture on impact. While oval stones may show a great demand, boulder opals often cut in free shapes can lend themselves to more distinctive designs.
• Marks: most opals, particularly black, have sand in the back of the stones. This usually does not affect value. However, marks or cracks that are fairly noticeable in the face of the stone will drastically affect value.

Looking after your Opal
Although opals are a magnificent and unique gem, they do not have the resistance to breaking or scratching that diamonds or sapphires have. If you protect the stone from heavy blows, and abrasive materials, and don‘t subject it to extreme sudden heat changes, your opal should last forever.
If you have an opal doublet or triplet, never immerse it in detergent (and preferably not even water), as this may break down the adhesive quality of the glues that have been used and cause separation (or ‘lifting’).