Opal Opals Boulder and Nut Specimens - Rough Search for a particular type or the number of an opal Australian Outback Other Fields Lightning Ridge etc. 2 .nH 2 O where the water content ranges from one to 21 percent, and between six and 10 percent for precious opal. There are two main forms of opal recognised - precious opal and common opal. That division is based on the presence or absence of the play of colours. Precious opal exhibits the characteristic play of spectral colours.
Common opal is mostly opaque and shows no play of colours.
When common opal is found in association with precious opal it is known as potch . Most opal is common - precious opal showing brilliant spectral colours is comparatively rare. This non-crystalline nature is unlike most other gemstones. Opal is really a hardened gel of hydrated silica. The background or body colour of the opal ranges from transparent through translucent and almost colourless to milky white, pale coloured to dark greys and black.