Opal

Class: Oxides & hydroxides

Regarded as mineral species for historical reasons; not a true mineral in the accepted sense of the word as it is either composed of cristobalite and/or tridymite or of amorphous silica; sedimentary and secondary from the alteration of high silica igneous extrusive rocks. There are specimens of 2610 carats (Arem, 1987: 139).

Opal — specimen 0211, photo © NMNHS

Original description: unknown [prehistoric].

Type locality: unknown [prehistoric].

Type material: unknown.

Etymology: from the Sanskrit upala, stone or precious stone.

Distribution: many localities world-wide.

Chemistry

SiO2·nH2O

Essential elements: hydrogen (H), oxygen (O), silicon (Si).

Crystal data

Crystallography: amorphous. Crystal habit: no crystalline forms; massive, botryoidal, reniform; replacement of other minerals or wood.

Physical properties

Cleavage: none (Arem, 1987: 139). Fracture: uneven, splintery, conchoidal. Tenacity: brittle (Arem, 1987: 139). Hardness: 5.5—6.5 (Arem, 1987: 139). Density: 1.98—2.25 g/cm3 (Lazzarelli, 2012). Luminescence: green or yellow, occasionally green-yellow, rarely yellow or red in SW.

Optical properties

Colour: colourless, white, yellow, orange, red, yellowish brown, greenish, blue, grey, black, violet. Diaphaneity: transparent to opaque. Lustre: vitreous, waxy, pearly, dull. Refractive index: 1.37—1.52 — isotropic (Lazzarelli, 2012). Dispersion: very low (Arem, 1987: 139). Pleochroism: distinct; red, orange, yellow, green, blue; see opalescence.

Material from ‘Repository’

40 specimens: 0376 — 0.27 ct, Mexico; 0417 — 0.16 ct, Mexico; 0167 — 1.15 ct, Mexico; 0412 — 0.36 ct, Mexico; 0211 — 1.60 ct, Mexico; 0227 — 1.66 ct, Mexico; 0544 — 1.62 ct, Madagascar; 0722 — 0.38 ct, Mexico; 0229 — 1.06 ct, Madagascar; 0264 — 1.58 ct, Madagascar; 0461 — 3.15 ct, Madagascar; 0469 — 0.19 ct, Ethiopia, Amhara, Semien Wollo, Wegeltena; 0518 — 2.37 ct, Madagascar; 0713 — 0.88 ct, Ethiopia, Amhara, Semien Wollo, Wegeltena; 0720 — 0.43 ct, Ethiopia, Amhara, Semien Wollo, Wegeltena; 0373 — 2.79 ct, Madagascar; 0557 — 1.99 ct, Madagascar; 0361 — 0.91 ct, Madagascar; 0073 — 0.59 ct, Madagascar; 0176 — 1.15 ct, Madagascar; 0749 — 1.07 ct, USA, Oregon, Malheur County, Lake Owyhee, 43.6248°N 117.23541°W; 0750 — 2.11 ct, USA, Oregon, Malheur County, Lake Owyhee, 43.6248°N 117.23541°W; 0756 — 1.76 ct, USA, Oregon, Malheur County, Lake Owyhee, 43.6248°N 117.23541°W; 0757 — 2.15 ct, USA, Oregon, Malheur County, Lake Owyhee, 43.6248°N 117.23541°W; 0409 — 0.75 ct, Peru; 0036 — 2.12 ct, Ethiopia, Amhara, Semien Wollo, Wegeltena; 0080 — 1.66 ct, Ethiopia, Amhara, Semien Wollo, Wegeltena; 0386 — 0.31 ct, Ethiopia, Amhara, Semien Wollo, Wegeltena; 0746 — 1.08 ct, Turkey, Eskişehir; 0747 — 2.24 ct, Turkey, Eskişehir; 0748 — 2.33 ct, Turkey, Eskişehir; 0365 — 0.60 ct, Ethiopia, Amhara, Semien Wollo, Wegeltena; 0273 — 1.72 ct, Madagascar; 0714 — 1.20 ct, Madagascar; 0715 — 0.71 ct, Madagascar; 0716 — 0.88 ct, Madagascar; 0717 — 0.84 ct, Madagascar; 0718 — 0.70 ct, Madagascar; 0721 — 0.45 ct, Madagascar; 0347 — 0.99 ct, Madagascar.

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