Solid Opal

The majority of opals are solid opals. A Lightning Ridge solid opal is all opal i.e. no non-opal material is attached. The opal may consist of all precious opal or a combination of common opal and precious opal. The most common categories are Black Opal, Dark Opal, Light Opal, Crystal Opal, Matrix Opal, Yowah-Nut and Boulder Opal. Dark Opals and Light Opals are also called Semi-Black Opals. Solid Boulder Opal consists of a fine layer of natural opal which has formed naturally on ironstone rock. Although there are two types of material combined in the same stone, the opal is a solid boulder opal.

Assembled Opal

Doublets are a piece of solid precious transparent or translucent coloured opal is glued to a dark backing (usually black potch), imitating natures solid stone. Triplets are also man-made and as their name implies, are made of three pieces of material - a thin layer of coloured opal, a cap of clear quartz and a black base of either potch or artificial material. Mosaic is another man-made way in which opal is presented. Small pieces of opal are fixed onto a dark backing and enclosed in resin.

Treated Opal

A naturally light or clear opal might be dyed to give it a dark background. This dark background will highlight the opal colors. The most common treated opal is Andamooka Matrix. Treatment of the limestone matrix involves a fairly simple process of simmering the stones in a very rich solution of sugar in water, followed by a period of simmering in a bath of very concentrated sulfuric acid. The sugar penetrates into the limestone matrix and then the acid 'cooks' it, turning the white sugar behind and around the pockets of opal into a black carbon.

Man-made Opal

These opals are artificially grown in a laboratory. The most common man-made opal are the Gilson opal and Slocum opal. Synthetic solid opal can be difficult to identify, unless you have seen a lot of opal. Look closely at the pattern - Opal created in a laboratory (Gilson opal), displays bright colours in large patches of colour. The pattern is often too 'perfect' and ordered, and can also often display a 'snakeskin' pattern. If you are still not sure, take it to a gemmologist or an opal expert. Present day manufacturers of synthetic opals include Pierre Gilson (the original manufacturer), Inamori (a Japanese manufacturer) and unspecified Russian and Chinese manufacturers.