Opal is a mineraloid because it is amorphous hydrated silica. Despite its mineral-like properties, opal's chemical structure is hydrated with varied water concentration. Because of this variance, the stone's structure isn't fixed or organized and thus, qualifies as a mineraloid.
Is opal a mineraloid?
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%. Because of its amorphous character, it is classed as a mineraloid, unlike crystalline forms of silica, which are classed as minerals.
Is opal amorphous?
Scientifically, opal is known as SiO2·nH2O – an amorphous mineraloid comprised of hydrated silicon dioxide. (A mix of silica and water). The opal's appearance can range from colourless, white, and light grey through to dark grey and black.
