Geological Classification
There are two ways of describing opal types by way of its geological formation. The first method has always been to describe opal by the name of the rock type that the opal is associated with. By this method we distinguish two and potentially three types of opal. They are “Volcanic opals”, and “Sedimentary opals” and are typically used in different types of literature. The third type of opal then is “Metamorphic opal”. This classification of opal can be misleading when it is used to discuss opal, as it is impossible for opal to survive in the volcanic geological environment. The temperatures of molten volcanic rocks (lavas or magmas) are far too high to allow for the formation of opaline silica. Because of these higher temperatures it is likely that the available silica would form in its normal crystalline polymorphs or varieties (for example, quartz or chalcedony) rather than opal. In the metamorphic environment similar constraints occur due to the effects of both temperature and pressure. Therefore, this classification relates only to an “association” that the opal has to rocks that have already formed and solidified over previous geological time. The second method of classification is the relationship of occurrence by geological structure.
Classification by association of rock types
Sedimentary opal is opal that is found in conjunction with sedimentary rocks such as sandstones and clay stones, for example at Lightning Ridge (NSW).
Volcanic opal is opal that is found in conjunction with volcanic rocks such as basalt, rhyolite or tuff, for example in Tintenbar (NSW).
Metamorphic opal is opal that is found in conjunction with metamorphic rocks such as schist, for example in Coolgardie (WA).
Classification by association of geological structure
Opal is often described by its mode of formation within the geological environment and consequently terms such as seam opal, “nobby” or nodule opal, nut opal, boulder opal, and pipe opal, are terms that have been widely used to describe opal.
Seam opal is opal that forms in thin layers in the strata
Nobby Opal is opal that forms in “nodules” in various sizes, (see discussion below on Lightning Ridge opal terms).
Pipe opal is opal that forms in small diameter vertical structures.
