Opal encountered in the sedimentary opal fields, especially of the type occurring in Australia, is clearly formed by colloidal processes. Precious opal itself is a special case of this process, and comprises only a very small fraction of the opal in these areas. The microstructure of small spheres is now well known, especially the regular stacking of uniform particles in the gemstone.

The key factor in these cases is the ultimate origin of the silica itself. In Australia, the opal deposits occur in soft siliceous claystones of Cretaceous age which are overlain by further siliceous deposits which have undergone intensive weathering. The mobilisation of silica is evidenced by the formation of silicified duricrusts and hard siliceous bands at various levels beneath the surface.

It is believed that in times of higher rainfall, the silica, both in true solution and as finely dispersed colloid, was carried downwards through the porous beds by percolating water. At a certain point the water encountered beds composed mainly of clay minerals which were much less permeable, especially to colloid particles.

This gave time for the primary particles of silica, perhaps 10-20 nm diameter, to coagulate or grow to larger sizes. Additional silica in solution would also be polymerised with the help of the abundant electrolytes, especially calcium and aluminium sulphates, which were also present in this environment.

Under the most favourable conditions of accumulation uniform spheres form and settle into a regular packed structure. Six sets of conditions would be necessary to achieve this:

	Quiescent conditions
	Uniform concentration of silica in the form of the primary colloid particles.
	Slow and uniform aggregation of these particles.
	Aggregation to a size suitable for optical diffraction effects to occur when solidified.
	Slow settling of the spheres under conditions to allow them to pack in a regular fashion.
	Slow removal of water, in the first place by absorption by underlying clay, and finally by slow drying.

It is clear that a combination of such sets of conditions would be rare; this is certainly confirmed by the limited occurrence of gem quality opal in the opal fields. Nevertheless, such conditions do occur occasionally over vast areas of country, especially that covering the Cretaceous beds of the Great Artesian Basin of Australia.

These basic conditions must also hold in other areas where precious opal deposits are found in what are essentially sedimentary environments. The important opal fields of the Pedro Secundo area of Piaui State in northeastern Brazil appear to have similar environmental conditions for the formation and deposition of opal.

While the actual host rocks are of volcanic origin, they are, in fact, brecciated and precede the formation of the opal. They have evidently been subject to later leaching, the silica having then been carried downwards and deposited in a similar manner to the opal in the Australian fields. This no doubt also applies to the American deposits at Rainbow Ridge, Nevada.