The body tone or base color of an opal is different to the play-of-colour displayed in precious opal. There are three varieties of natural opal based on body tone. Body tone refers to the relative darkness or lightness of the opal when ignoring the play-of-colour.
Black Opal - is the family of opal which shows a play-of-colour within or on a black body tone by reference to the AGIA Body Tone Chart N1, N2, N3 and N4 when viewed face up.
Dark Opal - is the family of opal which shows a play-of-colour within or on a dark body tone by reference to the AGIA Body Tone Chart N5, N6 when viewed face up.
Light Opal - is the family of opal which shows a play-of-colour within or on a light body tone by reference to the AGIA Body Tone chart N7, N8 or N9 when viewed face up. The N9 category is referred to as white opal. Opal with a distinct colored body (such as yellow, orange, red or brown) should be classified as black, dark or light opal by reference to the AGIA Body Tone Chart with a notation stating its colour hue.
Crystal Opal – Transparent to Semi-Transparent, Colorless Body with play of color.
Black Crystal Opal - Transparent to Semi-Transparent, Dark Body with play of color.
Fire Color - There are usually a dominant color and then a secondary color.
Natural opal is opal that has not been treated or enhanced in any way other than by cutting. There are three basic forms of natural opal described by visual appearance. Precious Opalexhibits 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 - Opal that does not exhibit a play-of-colour. The distinction between common opal and potch is based on formation and structure. Common opal shows some degree of micro crystallinity.
Natural Opal Type 1 - 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 - 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 or doublet opal (D.O)
The play of color is shown through veins that are quite thin, and thus there are new unusual methods of preparing the stone as a gem. An opal doublet is commonly set with a thin layer of opal, backed by a swart mineral such as ironstone, basalt, or obsidian. The darker backing emphasizes the play of color, and results in a more attractive display than a lighter potch.
Techniques - Modern techniques of polishing give doublet opals enhancement similar to the effects of black or boulder opals at a mere fraction of the price. Doublet opal also increases customer investment since it has an added benefit of having genuine opal as the top visible and touchable layer, unlike triplet opals.
Characteristics - A dark backing is placed behind the stone, and then has a domed cap of clear quartz or plastic on top, which takes a high polish and acts as a protective layer for the opal. The top layer also acts as a magnifier, to emphasize the play of color of the opal beneath, which is often of lower quality. Triplet opals therefore have a more artificial appearance, and are not classed as precious opal.
Natural Opal Type 3 - opal presented in one piece where the opal is intimately diffused as in-fillings of pores or holes or between grains of the host rock in which it was formed. This opal is commonly known as matrix opal.