DIRK WIERSMA / SCIENCE PHOTO LIBRARY DIRK WIERSMA / SCIENCE PHOTO LIBRARY
Opal is hydrated, amorphous silica (SiO2), deposited from silica-bearing waters in voids and fissures of certain rock formations. Water molecules trapped in the silica, together with interior conchoidal fractures, account for the milky lustre and brilliant pastel colours that make opal into such a desirable precious stone. When used in jewellery, its main application, opal is usually smoothly polished in the form of rounded cabochons, often of ellipsoid shape.
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