Often you may hear people talking about Black, Crystal, Boulder Opals, as well as Solids, Doublets and triplets. The information here is only a brief outline of the different types.
Solid Opal has been mined and is presented in one whole piece, either in its naturally occurring state or after being cut and polished. It has not been chemically treated and has no other materials cemented to it.Solid Opal can be put in water without worry.
Some of the different types of Solid Opal are:
Crystal Opal is a transparent Solid Opal with bright colours flashing through it. Crystal Opal is called "Crystal" because of its transparency, however to be of any value it must have natural colour, the more colour in the gemstone the more it is valued.
Black Opal is a naturally dark Solid Opal, Black Opal is not just black as the name suggests, it is a dark opal with bright colours flashing through it.
Boulder Opal is a bright Solid Opal that is usually cut with its natural Ironstone base left on, giving it more body. Boulder opal is not mined in Coober Pedy.
Matrix Opal is a beautiful Solid Opal that is a natural matrix/composite of opal and a mother stone which sometimes gives a speckled appearance. Matrix is often cooked in a solution creating a contrast in the mother stone allowing the natural colour of the opal to be seen.
Milky Opal is best described as White opal that is called Milky because of its pale colour and lack of intense bright colours.
The folowing are Not Solid Opals, however they do use Opal as the basis of the stone.
Doublets & Triplets
A "Doublet" is a thin layer of Solid Opal that is laminated to a base of dark material making up 2 layers.
A "Triplet" is essentialy a doublet with a clear cap of either glass or quartz making up 3 layers. The quartz top often magnifies the colour of the stone, also giving a protective cover for the thin piece of Opal.