Opal is a mineraloid, an amorphous variety of silica. Does not have a crystal lattice. Contains water, when it is lost it becomes cloudy. May have high decorative properties. Opals are characterized by both a variety of colors and complete transparency. In its best examples, it refers to precious stones.
You can see an opal stone for sale and buy it in the Minerals of Russia online store.
Colorless, white, yellow, red, orange,
green, brown, black, blue, pink
|Crystal habit||Irregular veins, in masses, in nodules|
|Fracture||Conchoidal to uneven|
|Mohs scale hardness||5.5–6|
|Luster||Subvitreous to waxy|
|Diaphaneity||opaque, translucent, transparent|
|Polish luster||Vitreous to resinous|
|Optical properties||Single refractive, often anomalous double refractive due to strain|
Mexican opal may read as low as 1.37, but typically reads 1.42–1.43
black or white body color: inert to white to moderate light blue, green, or yellow in long and short wave, may also phosphoresce, common opal: inert to strong green or yellowish green in long and short wave, may phosphoresce; fire opal: inert to moderate greenish brown in long and short wave, may phosphoresce
|Absorption spectra||green stones: 660 nm, 470 nm cutoff|
|Diagnostic features||darkening upon heating|
|Solubility||hot salt water, bases, methanol, humic acid, hydrofluoric acid|