A gem opal consists of monodisperse colloidal spheres of silica being arranged in orderly arrays. This must have been formed in the geological past from a colloidal suspension of silica and then be desicated. The formation process from the suspension would be a sort of the phase transition from a disordered to an ordered state which is seen in a monodisperse latex. This transition is characterized by a repulsive interaction between the particles, and recently identified as Kirkwood-Alder transition that is considered to be an essence of the liquid-solid transition.
The opal structure is the same as that in monodisperse latexes which can be seen under a light microscope. It is interesting that such structures reflect some aspects of the atomic structure in crystals. There are sometimes found, in multicomponent opals, superstructures such as AlB2-and CaZn13-type and these are also found in binary mixtures of monodisperse latexes.
Now colloid science is opening a new aspect in the investigation of the structure of alloys and some compounds.
