The Opal

Opal is an amorphous quartz composed of silica anhydride and water, SiO² + H²O, being a colloidal substance. It does not have a crystalline structure and lacks a regular shape. Opal generally appears in the form of mammillary, reniform, or stalactite-like incrustations. It is monorefractive, with a low refractive index that can sometimes be abnormal, birefringent. It has a vitreous luster, tending towards grease, resin, or wax. Perfect transparency is found only in the hyalite variety, and its density is much lower than that of crystalline quartz, resulting in less hardness, but the optical properties are not so different.

Opal lacks a crystalline structure, although it possesses a minimum of reticular order with variable entities. It is very fragile; if it loses water quickly, Opal can develop tiny conchoidal fractures. Its vitreous luster is weak, opalescent, and often milky. In the translucent white-blue varieties, it is opalescent and iridescent due to refraction, possibly due to a fine lamellar structure or a microstructure of internal silica spheres (found in precious Opal, absent in our environment). In the semi-opal variety, it ranges from brownish to black, even in a highly acidic environment.

In general, Opal represents a metastable phase of chalcedony. This means that it slowly transforms into a crystalline state, losing water in the process. All of this happens with natural aging over relatively short geological time periods. Poor Opal likely originates from silica with a different history, transported directly from below by later hydrothermal fluids rich in silica.