Opal is a mineraloid - hydrated amorphous form for silica. The Russian name is adapted from the Latin "opalus" is the name of this mineral, distorted from the Sanskrit "उपल (upalaḥ)" - "stone". Opals have a variety of colors from black and white to yellow, red, pink, green, blue and violet.

Chemistry: SiO2•nH2O;

Crystal system: none, amorphous;

Color: "Common" opal, which is essentially a kind of rock, displays play-of-color from colorless and white to black and from red to purple. "Precious" opal shows a not so variable play colors: additional colors (opalescence). The main colors of "precious" opal are colorless, white, gray, black, brown, yellow, orange, and red. Additional colors are shown in opalescence - from blue to red. The main color is due to the saturation of opal chromophore impurities - mineral inclusions, salts of iron, manganese, chromium, nickel, copper and others. Additional colors ordered silica spheres produce the internal colors by causing the interference and diffraction of light passing through the microstructure of the opal. The regularity of the sizes and the packing of these spheres determines the quality of precious opal. The colors that are observed are determined by the spacing between the planes and the orientation of planes with respect to the incident light.

Identification properties

Physical properties
Mohs hardness: 5.5 – 6.5
Density: 1.75 – 2.25 g/cm3
Cleavage: imperfect
Fracture: Uneven, even, conchoidal
Optical properties
Optical character: isotropic
Refractive Index: 1.370 – 1.470
Birefringence: none
Pleochroism: none
Dispertion: very weak
Luster: vitreous

Inclusions and structural inhomogeneities