Different types of opal have been recognised, largely by their outward appearance, by the mineralogist and collector for 100 years or more. More recently it has been recognised that there are certain categories into which opals can be placed by virtue of their crystal structure, or lack of it. Fundamental differences were first suspected in 1963 when Dr. Jones and I found that there were major variations in differential thermal analysis patterns of various kinds of opal which suggested that there could be some fundamental differences at least in their micromorphology (R0371).

The X-ray diffraction (XRD) work showed that there were at least three distinct categories into which opals could be placed. These were discussed in more detail in the paper published in 1971 by the same authors (R0242). The three types of XRD patterns obtained from a large number of opal specimens from various parts of the world are illustrated (B, C-D, and E).

In general, there was no difficulty in classifying an opal into one or the other of these three groups; less than two percent of opals examined appeared to be transitional forms.

The opal represented by pattern type B is close to that of well crystallised low cristobalite, although more recent detailed work by Graetsch and co-workers (R1290) indicates that there is a considerable degree of disorder in the structure.

It was suggested that this type of opal be referred to as opal-C. This turned out to be a rather rare type, being confined in the original work to a small number of samples from volcanic rocks.

It is thought that, at least in some cases, the opal was originally formed as one of the other types and heated at a later stage by further volcanic activity. Laboratory tests have shown that opal can be converted to cristobalite on heating to temperatures above about 800°C.

The second group, which gave patterns similar to those in categories C and D were the most common. They gave a strong XRD pattern, with the major peaks in the region of those for cristobalite and tridymite. The patterns suggested that the underlying structure was of a disordered stacking of cristobalite and tridymite atomic layers (more detail is given in the section Atomic Structure) It was found that most common opals fell into this category, including some of the earthy bedded deposits of opaline silica. It was suggested that these types of opal be termed opal-CT.

The third group yields patterns similar to E. Apart from a broad maximum in the region of approximately 4Å footnote, there were no other diagnostic characteristics. While it was found that long exposure X-ray powder photographs gave several broad bands, there was little to suggest any appreciable order in the structure; it was amorphous or near amorphous in character.

It was found that most precious opal and its associated potch fell into this third category; exceptions were some volcanic precious opals. A few common opals also fell into this category, but these were rare. In addition, most biogenic material such as diatomite and plant phytoliths fall into this category. The suggested terminology for this group was opal-A.

More recently it has been pointed out by Langer and Flörke (R1551) that the silica in the glass-like form of opal, hyalite, has a different structure to that of precious and related opals. They suggest dividing the opal-A category into opal-AG and opal-AN, the last named to cover the special category of hyalites.

This classification is simple and most opaline materials fit easily into it; since its publication, it has been widely adopted. The final classification suggested by Jones and myself, with the inclusions of Flörke and Langer's suggestions, is summarised.