Australian opal definitions and terms

  • OPAL is a hydrated amorphous form of silica with a chemical formula SiO2nH2O. It is a mineraloid.   

  • PRECIOUS OPAL exhibits a 'play of colour', whereas 'common opal' or 'potch' does not. Here we just refer to precious opal as opal.

  • NATURAL OPAL is opal that has not been treated or enhanced in any other way other than by cutting and polishing. Also known as natural solid opal.

  • BODY TONE is the base or the background colour of the opal. It is assessed on a grey scale (i.e. light to dark). 

  • BLACK OPAL has two different definitions.

    1. Natural opal that has a black body tone or background.

    2. Natural opal that is found in one piece where the opal is in its natural state and is of substantially homogenous chemical composition. It should also have a black or dark
    body tone.

  • PLAY OF COLOUR is defined as a pseudochromatic optical effect resulting in flashes of coloured
    light from certain minerals or mineraloids, as they are turned in white light. In other words, the way that colours change within a particular stone as it is rotated and tilted.

  • BOULDER OPAL is natural opal that is from the opal mining fields in Queensland, Australia. It is found in one piece where the opal is naturally part of the host rock in which it was formed.

  • MATRIX OPAL is a term to describe natural opals found in one piece where the opal is intimately infused as infillings, pores or holes or between grains of the host rock in which it was formed.

  • ROUGH BOULDER OPAL is boulder opal as it comes out of the ground, with the excess host rock removed where possible. It has not been touched by lapidary equipment. It contains evidence of opal as seen by opal ‘veins’ running through the host rock. The host rock may be sandstone or ironstone. It requires bigger cutting equipment and especially larger saw blades to work with rough boulder opal. It is also extremely heavy.

  • WOOD REPLACEMENT OR PIPE OPAL is a term to describe natural opals that forms in pipe-like structures. This unique type of boulder opal is usually found in softer clay or sandstone, in contrast to opal found in harder ironstone or sandstone boulders. For an opal to be termed wood replacement opal, it is hypothesised that a wood branch or tree root has become opalised.

  • BOULDER OPAL SPECIMENS are where the ‘face’ of the opal is kept in its natural state in larger pieces of their host rock. There can be three variations; (i) both the opal and host rock are polished (ii) only the opal is polished and (iii) the opal and host rock is not polished (i.e is raw / rough).

  • BOULDER OPAL GEMSTONES are natural boulder opals that have been cut and polished. They are exclusively found in Queensland, Australia.

  • CRYSTAL OPAL is any kind of opal that has a transparent, translucent or semi-translucent. In other words, it will permit the passage of light.