The OPAL is a resinous quartz, which owes its beauty to its imperfections. Its milky and slightly bluish and semi-transparent ground is coloured by all the hues of the rainbow, resulting from fissures which interrupt the continuity of the substances.

The opal is highly esteemed in commerce; it is cut either in "cabochon" or "goutte de suif," and is even engraved. The opal matrix is cut into slabs, tazze, or snuff-boxes.