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.
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.
NATURAL OPAL (also called natural solid opal) is opal that has not been treated or enhanced in any other way other than by cutting and polishing.
BLACK OPAL has two different definitions:
- 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.
- Natural opal that has a black body tone or background.
BODY TONE is the base or the background colour of the opal. It is assessed on a grey scale (i.e. light to dark).
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.
BOULDER 'OPAL RUBS' are rough boulder opal pieces that has been split and/or been worked with to expose the opal present. Boulder opal rubs are only partially worked. More opal may need to be exposed using diamond grinding wheels. They have not been given their final shape and polish.
BOULDER OPAL SPECIMENS are where the ‘face’ of the opal is kept in its natural state in larger pieces of their host rock. There are typically two variations; (i) both the opal and host rock are polished and (ii) only the opal is polished.
BOULDER OPAL GEMSTONES are natural boulder opals that have been cut and polished. They are exclusively found in Queensland, Australia.