Opal

Opal is a hydrated amorphous form of silica; its water content may range from 3 to 21% by weight, but is usually between 6 and 10%. Because of its amorphous character, it is classed as a mineraloid, unlike crystalline forms of silica, which are classed as minerals. Opal is an opaque stone, with a see through surface and some minute tinges of Colours in it that give it its famous firing.