Opal is a hydrated amorphous form of silica; its water content may range from 3 to 21% by weight, but is usually between 6 and 10%. Because of its amorphous character, it is classed as a mineraloid, unlike the other crystalline forms of silica, which are classed as 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.
|Color||Colorless, White, Yellow, Red, Orange, Green, Brown, Black, Blue|
|Mohs scale hardness||5.5 - 6.5|
|Specific gravity||1.98 - 2.25|
|Refractive index||1.440 - 1.460|