Opal-AG

A variety of Opal

This material should properly be labelled as Opal-A_G.

A type of Opal consisting of aggregated spheres of amorphous silica, with water filling the gaps in between. Precious Opal and Potch Opal consist of this kind of structure - the difference being in the regularity of the sizes of the spheres and packing.

"Precious Opal" shows a play of colours in white light that is due to diffraction from the regular packing of the clathrate-like silica gel spheres. Although the packing of the spheres may be regular, there is neither short-range nor long-range order in this material, and the X-ray powder diffraction pattern is characterized by a distinct broad hump and a possible weak second hump indicative of material that is "X-ray amorphous". The subscript G has been added to indicate that the structure of the silica network is gel-like in that it is composed of large cages with included water essential to the stability of the structure. Most but not all precious opal is opal-AG.

Opal that shows no play of colours is also composed of the same spherical clusters, but they are non-uniform in size and do not pack in an orderly manner thus destroying any possible diffraction of the light. This type of opal is termed collectively "potch opal" and includes the massive varieties. Porous varieties of inorganic origin such as geyserite and materials composed of tests of micro-organisms such as diatomite and radiolarite are also recognized. The X-ray powder diffraction pattern is essentially identical for of all these types of opal regardless of their origin.