A natural hydrated form of silica. Opal is a relatively common mineral in its nongem form, which is known as common opal and lacks the play of colour for which gem, or precious, opal is known. All opal is of relatively simple chemical composition,SiO₂ · nH₂O, in which SiO₂ represents silicon dioxide, and n represents a variable amount of water (H₂O) that is housed in the internal structural arrangement of the mineral. The water content generally ranges from about 4 to 10 wt %, but it may be as high as 20 wt %. The hardness of opal on the Mohs hardness scale ranges from 5 to 6, the specific gravity from 2.25 to 1.99, and the refractive index from 1.455 to 1.435. Both specific gravity and refractive index decrease as a function of increasing water content.