However, if the packaging is very heterogeneous, then the interference of diffracted beams colors do not appear. Painting of opals monotone milky-white, so they are called ordinary. Precious opal, as well as other types of opaline silica formed in the surface or subsurface conditions. Opal silica is deposited as a result of biochemical processes that form the skeletons of diatoms, sponges, radiolarians, and other organisms, or is formed by the deposition of ionic or colloidal solutions. The solubility of amorphous silica or kristobatolitovogo in water under normal conditions is much higher than the solubility of silica, increasing sharply at a pH greater than 9, as well as the temperature rises. Natural thermal water is often alkaline composition enriched in silica in contact with rocks containing silicate minerals. Opal - the most contaminated with various impurities and less-ordered crystal structure of the mineral silica. Therefore, its genesis is usually explained by rapid crystallization from supersaturated solutions in an environment rich in extraneous ions. Also suggest that the silica falls out of solution in the form of a gel, which is aging, and other forms opal-temperature silica minerals. In nature, the noble opals are extremely rare, indicating that very specific conditions of their formation. The homogeneity of the structure of precious opal may be due to persistence over a certain period of time constants of physico-chemical and thermodynamic parameters of mineral-forming medium. In some cases, an ordered internal structure of precious opal is inherited as a result of replacement of silica remains of flora and fauna.