Opal is hydrated, amorphous silica (SiO2), deposited from silica-bearing waters in voids and fissures in many types of rock formations. Water molecules trapped in the silica, together with interior conchoidal fractures, account for the milky lustre and wondrous pastel colours that make the higher quality opal into a desirable precious stone for jewellery. Actual height 95 mm. Provenance is north eastern Oregon, USA.