SiO2.nH2O
Colour: Colourless (hyalite) or white, but also grey, brown, or red, the colour being usually due to fine grained impurities; also with a rich iridescence and play of colours (precious opal)
Streak : White
Hardness: 5.5 - 6.5
Massive, often in rounded and botryoidal forms, sometimes pisolitic. Opal has a vitreous or waxy lustre.
Opal is distinguished from chalcedony (which it resembles) by its low density.
