Opal

Opal
A rich seam of iridescent opal encased in matrix
General
Category	Mineraloid
Formula (repeating unit)	Hydrated silica. SiO2·nH2O
IMA symbol	Opl[1]
Crystal system	Amorphous[2]
Identification
Color	Colorless, white, yellow, red, orange, green, brown, black, blue, pink
Crystal habit	Irregular veins, in masses, in nodules
Cleavage	None[2]
Fracture	Conchoidal to uneven[2]
Mohs scale hardness	5.5–6[2]
Luster	Subvitreous to waxy[2]
Streak	White
Diaphaneity	opaque, translucent, transparent
Specific gravity	2.15+0.08 −0.90[2]
Density	2.09 g/cm3
Polish luster	Vitreous to resinous[2]
Optical properties	Single refractive, often anomalous double refractive due to strain[2]
Refractive index	1.450+0.020 −0.080 Mexican opal may read as low as 1.37, but typically reads 1.42–1.43[2]
Birefringence	none[2]
Pleochroism	None[2]
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[2]
Absorption spectra	green stones: 660 nm, 470 nm cutoff[2]
Diagnostic features	darkening upon heating
Solubility	hot salt water, bases, methanol, humic acid, hydrofluoric acid
References	[3][4]

Opal is a hydrated amorphous form of silica (SiO₂·nH₂O); its water content may range from 3% to 21% by weight, but is usually between 6% and 10%. Due to the amorphous chemical structure, it is classified as a mineraloid, unlike crystalline forms of silica, which are considered minerals. It is deposited at a relatively low temperature and may occur in the fissures of almost any kind of rock, being most commonly found with limonite, sandstone, rhyolite, marl, and basalt.

The name opal is believed to be derived from the Sanskrit word upala (उपल), which means 'jewel', and later the Greek derivative opállios (ὀπάλλιος).

There are two broad classes of opal: precious and common. Precious opal displays play-of-color (iridescence); common opal does not.^[5] Play-of-color is defined as "a pseudo chromatic optical effect resulting in flashes of colored light from certain minerals, as they are turned in white light."^[6] The internal structure of precious opal causes it to diffract light, resulting in play-of-color. Depending on the conditions in which it formed, opal may be transparent, translucent, or opaque, and the background color may be white, black, or nearly any color of the visual spectrum. Black opal is considered the rarest, while white, gray, and green opals are the most common.