Opal mineral - You can buy these samples

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.

Physical properties of Opal

General
Category	Mineraloid
Formula (repeating unit)	Hydrated silica SiO₂·nH₂O
IMA symbol	Opl
Crystal system	Amorphous
Identification
Color	Colorless, white, yellow, red, orange, green, brown, black, blue, pink
Crystal habit	Irregular veins, in masses, in nodules
Cleavage	None
Fracture	Conchoidal to uneven
Mohs scale hardness	5.5–6
Luster	Subvitreous to waxy
Streak	White
Diaphaneity	opaque, translucent, transparent
Specific gravity	2.15+0.08 −0.90
Density	2.09 g/cm³
Polish luster	Vitreous to resinous
Optical properties	Single refractive, often anomalous double refractive due to strain
Refractive index	1.450+0.020 −0.080 Mexican opal may read as low as 1.37, but typically reads 1.42–1.43
Birefringence	none
Pleochroism	None
Ultraviolet fluorescence	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