AUSTRALIAN OPALS

What is Opal? Opal, from the Greek 'Opallos' meaning 'to see a change (of colour),' is a formation of non crystalline silica gel which, 15-30 million years ago, seeped into the crevices of the host stone and through the effects of heat and pressure hardened to form this brilliant gem. The colourful display is the result of light passing through an array of tightly packed minute spheres of silica and being refracted at different angles into the colours of the spectrum. The size of the spheres determines the colour seen; the larger being less common and reflecting the red end of the spectrum. Hence, opals displaying red in the play of colour are regarded as more valuable. Common opal is the variety that displays no play of colour and is referred to as 'potch'.

Australian opal represents 95% of the world's total production. However, the amount of opal found is infinitesimal by comparison with other precious gemstones. For this reason, the rarity of the opal is much greater than is generally appreciated. There are three types of precious opal found in Australia: - the Boulder, the Light (or white) and the Black. Each has its own distinct characteristics and occur in different regions of the country.

Where Opals are found in Australia

What Factors Determine an Opal's Value?

There are four factors that determine the value of an opal:

1. The type of opal;	2. The play of colour displayed;
3. The pattern of colour patches.	4. Its brilliance

Generally black (or dark) opal is regarded as the rarest and most precious variety followed by boulder and light (with the exception of the crystal variety). The greater the range of spectral colours visible the more precious the opal ie: an opal displaying violet through to red is rarer than one only showing blue-green colours. There are several different patterns including the harlequin and pinfire that are the most sought after varieties.