Gemmology
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Opal — specimen 0717, photo © NMNHS
Opal — specimen 0717, photo © NMNHS

Opalspecimen 0717

Origin

East Africa
Madagascar
Opal — specimen 0717, locality map
Data

Weight: 0.84 ct; size: 7.39 | 6.49 | 4.56 mm; shape: hexagon; colour: light grey; clarity: eye clean; cut: excellent; treatment: none.
Notes

Very clean specimen; design ‘Hexamixtum S 41-32’ [pavilion angle changed to 45.00], excellent cut by Tan Pen.

Other specimens
Opal — specimen 0376Opal — specimen 0417Opal — specimen 0167Opal — specimen 0412Opal — specimen 0211Opal — specimen 0227Opal — specimen 0544Opal — specimen 0722Opal — specimen 0229Opal — specimen 0264Opal — specimen 0461Opal — specimen 0469Opal — specimen 0518Opal — specimen 0713Opal — specimen 0720Opal — specimen 0373Opal — specimen 0557Opal — specimen 0361

More information from ‘Classification’


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).