Many people are unaware that Opal, one of October's birthstones, is Australia 's national gemstone. Australia not only mines 95 per cent of the world's precious black and white opal but offers opals of many varieties used in jewellery, including milky opal, boulder opal, crystal opal and some fire opal.
The most common and affordable variety is milky, white or light opal because it shows a play of colour against a white opaque background. Colour patches or tiny flashes called 'pinfire' are usually light and bright pinks and greens.
Crystal opals are transparent to translucent, with a subtle sheen of colour dancing through the gem, rather than colour patches.
The most valuable opals, known as black opals, feature large, luminescent areas of one or more bright colours against a dark background. Opals are rarely treated to enhance their colour, however they can be quite delicate and should not be exposed to steam cleaning or excess acids. The best cleaning method is with a soft damp or dry cloth.
Opal has a long history, dating back to the pre-Roman times, milky opals with patches of pastel red, blue and green were mined in what is now Hungary. The more familiar black opal, with brilliant flashes of red, blue, green and gold, was not discovered until the late 19th century in Australia. Today opals are also mined in Mexico, Brazil, United States and Canada.
Play of Colour. An opal's beauty is unique and made from the amazing 'play of spectral colours' in the gemstone. This originates from the breaking up of white light due to the three dimensional spherical shaped microstructure of the silica particles inherent in the opal (these can only be detected with an electron microscope). Similar phenomena of colour hues can be observed when oil lies on water, or in a rainbow.
Formation. Opal occurs where silica gel fi lls small fi ssures or voids in the earth. A large portion of Australia's inland was once covered by a sea, leaving deposits and creating an environment suitable for the formation of opal.
Types of Opals
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 ie. 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 fields of Mintabie, Coober Pedy and Andamooka, although the first material was mined at White Cliffs (1887). This opal has a light body tone.
Crystal Opal. This variety embraces opal which is transparent or 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. This type, composed of opal naturally occurring on its host rock, is mined predominantly in Queensland. It is easily identified because, when cut, the host rock (a brown ironstone) is left on the back of the opal. Boulder opal may be light, dark or black. In the last twenty years this type of opal has become extremely popular as it can display the same darkness and brilliance as a high quality black opal.
Matrix Opal. There are two common types of Matrix Opal 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, a sandstone which is often porous and can be treated, turning the material dark resembling black opal.
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 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 fl at surface. While oval stones were traditionally in demand, Queensland Boulder Opal is now generally cut as a free form shape that lends itself to more distinctive designs.
Marks. Most opals, particularly the black opal, have sand in the back of the stones. This usually does not affect value. However, marks or natural inclusions that are fairly noticeable in the face of the stone could affect the value.