Opal and opaline silica occur in nature in a wide variety of forms, both structural (atomic) and physical (morphological). The basic structural classification into opal-AG, opal-AN, opal-CT and opal-C is indicative of different modes of deposition. This is further complicated by both the wide range of environments in which opal is formed, and the variety of microstructures found, especially in opal-CT. Even an homogeneous form of opal such as hyalite (opal-AN) may have more than one type of origin. The genesis of the opal also involves the solution and/or colloid chemistry of silica, but in the case of biogenic silicas, its formation is, at least in part, controlled by the metabolism of living cells.

The aqueous chemistry of silica has been intensively studied. The variation of solubility of silica in water in relation to the pH has been studied in detail, especially by Krauskopf (R1444). The solubility at 25°C is low (between 100 and 200 parts per million - ppm) up to about pH 9.5, but rises rapidly to about 5000 ppm at pH 10.5. At higher temperatures and pressures the classic work on the solubility of silica in water was no doubt that of Kennedy (R1597). Earlier work, which showed rather higher solubilities, was rendered suspect by the failure to ensure that all colloidal silica was filtered out during extraction of samples from the pressure vessels.

Kennedy systematically studied the solubility of silica in water between 200°C and 360°C at 300 atmospheres pressure, with some experiments up to 500°C and 1000 atmospheres pressure. The object of this work was to help to interpret the movement of silica under the physicochemical conditions met with at various depths in the earth's crust.

It is worth noting Kennedy's comment that:

This experimental work showed that the solubility of silica in water at 300 atmospheres pressure reached a maximum of some 110 parts per million at 360°C, decreasing at higher temperatures. We may therefore consider briefly some possible origins of various types of opaline silica found in nature:

	Geyserite and siliceous sinter: opal deposited from hot water, as for example, hot springs, usually opal-AG.
	Hyalite: opal deposited from solution/colloid at supercritical conditions, usually opal-AN.
	Common opal, usually opal-CT, deposited directly, or formed by diagenetic processes.
	Opal-AG, of the type found in precious opal deposits, formed by colloidal deposition and usually found in sedimentary environments.
	Precious opals from volcanic rocks.