Opals are composed of hydrated silica, and can contain up to twenty percent water by weight. The material is usually sub-classified as either “precious” or “common”, depending on whether or not its surface is able to display the iridescent play-of-color effect for which the species is best known. In the case of common opals, this phenomenon is not present and such stones are usually discarded if they do not possess unusual or attractive visual characteristics.

When they are retained and sold, common opals are given a wide range of varietal names depending on their inclusions, body color and/or degree of transparency. These include  “liver opal” (brownish), “prase opal” (yellowish-green and somewhat translucent), “jasper opal” (reddish-brown and opaque), “moss/dendritic opal” (contains plant-like inclusions) and “wax opal” (yellowish, with a waxy luster).