opal Hydrous silica SiO₂.nH₂O associated with the chalcedonic (see CHALCEDONY) varieties of silica. A layer of water molecules trapped near the mineral surface causes the iridescence (opalescence) which is a diagnostic property of opal; sp. gr. 1.99–2.25; hardness 5.5–6.5; amorphous; colourless, or milky-white to grey, red, brown, blue, green, to nearly black; resinous lustre; normally massive, but can be stalactitic, botryoidal, and also in veinlets, the various varieties depending on the amount of water contained in the mineral, which can vary from 6% to 10%; no cleavage; conchoidal fracture; normally deposited at low temperatures from silica-bearing waters, and occurs as fissure fillings in rocks of any kind, and especially near geysers and hot springs. The variety known as precious opal has a milky-white or sometimes black body colour which exhibits a brilliant play of colours, usually blues, reds, and yellows. The colours can often disappear with the loss of water when the mineral is exposed to air.

o·pal / ˈōpəl/ • n. a gemstone consisting of hydrated silica, typically semitransparent and showing varying colors against a pale or dark ground.ORIGIN: late 16th cent.: from French opale or Latin opalus, probably based on Sanskrit upala ‘precious stone’ (having been first brought from India).

Opal

Gemstone of quartz or silica, praised by Pliny the Elder (ca. 23-79 C.E.), who wrote: "For in them you shall see the living fire of the ruby, the glorious purple of the amethyst, the green sea of the emerald, all glittering together in an incredible mixture of light." In ancient times many legends existed around its claimed virtues. It was believed to recreate the heart, ward off airborne contagions, and dispel sadness. It was also good for weak eyes. The name poederos, applied to the opal, refers to the complexion of youth.

The superstition that opals were unlucky seems to have been popularized by Sir Walter Scott's novel Anne of Geirstein (1829). The story claims the opal worn by Baroness Hermione of Arnheim lost its luster after a drop of water touched it.

opal a gemstone which is typically semi-transparent and showing many small points of shifting colour against a pale or dark ground; it is often referred to allusively to evoke the idea of changing colours.

The belief that opals are unlucky is recorded from the 19th century, and may originate with Walter Scott's novel Anne of Geierstein (1829), in which an opal brings ill fortune on its owner.

opal Non-crystalline variety of quartz, found in recent volcanoes, deposits from hot springs and sediments. Usually colourless or white with a rainbow play of colour in gem forms, it is the most valuable of quartz gems. Hardness 5.5–6.5; r.d. 2.0.