Ópalo

Opal is a hydrated silica mineraloid (SiO2·nH2O) lacking a crystalline structure, formed in low-temperature settings.

It is deposited in cavities of volcanic rocks by cool hydrothermal solutions, where silica precipitates in layers and coats surfaces. It also forms by sedimentary and diagenetic accumulation of silica in lacustrine environments, producing compact botryoidal masses. Water content is variable and controls physical and optical behaviour.

The hyalite variety is amorphous, glassy, globular, lacks play of colour, and typically fluoresces green under UV. Fire opal shows variable transparency with internal flashes in green, red and orange hues. Precious opal displays iridescent play-of-colour caused by diffraction in ordered silica microspheres. Ethiopian opals often show vivid colours but are sensitive to changes in humidity.

Some common, non-precious lacustrine forms grow as rounded aggregates that resemble figures and are known as ‘idolillos’. They derive from biogenic silica deposits associated with diatoms. There are references to prehistoric use in ritual contexts and as amulets.

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