Precious opal in its pure form is colorless, but usually colored finely dispersed mineral admixtures containing iron, in various shades of yellow, brown, red, green, gray, blue and black. The black coloration due to the presence of carbon (graphite), and green-nickel compounds in the clay mineral impurities. Colorless Opal in the laboratory can easily dyed various coloring agents. Painting under the influence of opal-ray does not change. By itself, opals can be transparent, translucent or opaque. Brilliance of his glass, dull or oily, no cleavage, brittle, conchoidal fracture. The density, depending on water content and porosity ranges from 1.99 to 2.25 g/cm3. The hardness of opal from 5 to 6.5. Optically isotropic, it is sometimes observed weak anomalous birefringence. The refractive index varies from 1.435 to 1.455, depending on water content and amount of impurities. On the air in some precious opals as a result of dehydration cracks that devalues the stone. To prevent this, opals are sometimes kept in the ground for two or three years, allowing them to lose moisture slowly. Precious opal from ancient times been appreciated for its rich internal play of colors. Already in the XIX century. Several hypotheses have been proposed to explain the nature of color play opal interference and diffraction of light rays. In 1845 D. Brewster wrote that the color plots in disgrace associated with a uniform distribution of tiny pores that form a space lattice, and the different dimensions of this lattice are responsible for the appearance of color variety. Ayui believed that the color game is due to reflection of light from properly located internal cracks and bubbles. BERENDS in 1871, was linked to its reflection of light from a curved thin layer of opal that differ in refractive index. E. Bayer explained the presence of the color game relic of a layered structure, inherited from opal replacement of them finely twinned calcite grains. Later, S. Rahman suggested that the color is the result of the game in the correct sequence of layers of opal cristobalite and tridymite.