OPAL CLASSIFICATION.
Opal is a gemstone consisting of hydrated amorphous silica with the chemical formula SiO2.nH2O. There are two basic forms of opal described by visual appearance.
Precious Opal – is opal which exhibits the phenomenon known as play-of-colour, produced by the diffraction of white light through a micro-structure of orderly arrayed silica spheres to produce changing spectral hues.
Common Opal and Potch – is opal which does not exhibit a play-of-colour. The distinction between common opal and potch is based on formation and structure. Potch is structurally similar to precious opal but has a disorderly arrangement of silica spheres. Common opal shows some degree of micro crystallinity.
TYPES OF NATURAL OPAL
Natural opal is opal which has not been treated or enhanced in any way other than by cutting and polishing. There are three types of natural opal, with varieties described by the two characteristics of body tone and transparency.
Natural Opal Type 1 – is opal presented in one piece in its natural state apart from cutting or polishing and is of substantially homogenous chemical composition.
Natural Opal Type 2 – is opal presented in one piece where the opal is naturally attached to the host rock in which it was formed and the host rock is of a different chemical composition. This opal is commonly known as boulder opal.
Natural Opal Type 3 – is opal presented in one piece where the opal is intimately diffused as infillings of pores or holes or between grains of the host rock in which it was formed. This opal is commonly known as matrix opal.
