Opal comes from the indian Sanskrit word for stone. All Opals contain water between 3 and 30% and when this dries out it maybe possible to restore the colour by saturation in oils or epoxy resin or water will in some cases restore the Opal to former glory. Opal is also sensitive to knocks and damage and if has been heated easy to dry out and crack. Another form of treatment common to Opal is doublet or triplet Opals which are a small thin peice of Opal backed by another peice of material to strengthen it then if a triplet a third clear peice is placed over the top to to seal the Opal.
It has a mohls hardness scale of 5.5-6 and a refractive index of 1.37-1.52.