At Tintenbar, also in New South Wales, jelly-like precious opal was found to occur in a similar manner. This sample was opal-CT, suggesting that somewhat higher temperatures of deposition encourages actual crystallisation of the opal. At the other extreme were small cavities in the similar altered trachyte from Maleny, Queensland. Here, small pockets, a few mm across, were filled with glass-clear opal which showed only one colour flash across the pocket. In other words, the whole cavity was filled with uniform spheres in uniform packing.

In a few cases, X-ray diffraction patterns of opal are close to that of well crystallised cristobalite; these are the type designated opal-C. These are most likely to be found in or associated with volcanic rocks, so that it is postulated that they have been formed by the silica crystallising at a somewhat higher temperature during the deposition process. It has been shown experimentally by Flörke and co-workers (R1057) that cristobalite, as well as opal-CT, can be formed by hydrothermal deposition; the crystal structure of these products closely resembled the natural products.

In some cases it is possible that volcanic opal has been recrystallised by subsequent heating, such as further lava flow.

Jones and the author (R0242) have shown that heating in air converts both opal-A and opal-CT into cristobalite. The reaction is both time and temperature dependent, with opal-A converting more readily than opal-CT. It was found that crystallisation could be induced at least as low as 900°C given heating times of several weeks; given sufficient time, conversion may well occur at lower temperatures under natural conditions in, for example, a solidified but still hot lava.