Varieties of natural opal

The variety of a natural opal is determined by the two characteristics of Body tone and Transparency.
Depending on the specific Body tone and Transparency of an opal it is classed as one of the following varieties:

* Body tone is not applicable to almost colourless, transparent opals (explanation see Body tone)

Note: This classification is manly used with Solid Opals (Type 1). Opals of Type 2 and Type 3 are usually opaque. Therefore the Transparency results in no further subdivision of these two types.
The Body tone may sometimes be applied to Boulder Opals (Type 2) to emphasise that it has a particular black body tone. In this case the term BOULDER BLACK OPAL may be used.
Light Opals with a white body tone (N9) are sometimes called WHITE OPALS.
 

The outlined varieties above describe the most important varieties of Australian Opals. Sometimes the term JELLY OPAL is used for Crystal Opals that have a particular jelly-like appearance and only show a diffuse play-of-colour instead of the clearly defined and structured colour patches of other opals.

There are further varieties some of which may exhibit a special behaviour like CONTRA LUZ OPAL - meaning "against the light". This variety comes from Mexico and Oregon and shows its play-of-colour only by transmitted light and not by incident light as usual.
Another special variety is HYDROPHAN OPAL which is porous and almost opaque without a distinct play-of-colour in the dry state but when immersed in water the porous opal starts to soak up water, becoming translucent to transparent and starts to exhibit a play-of-colour. This variety occurs in Mexico and other places.