Reference work entry
Opal is a gem-grade mineraloid of hydrous silicon dioxide. Its name comes from the Latin ‘oplus’, meaning ‘the beauty of gem in one body’. Its chemical composition is SiO 2· nH 2O, and it can be opaque, translucent or transparent with a waxy lustre. It has a Mohs hardness of 6–6.5 and can be white, yellow, red or black. Its main characteristic is an iridescent colour, which stems from the silicon spherical structure that flashes with a diffraction grating. In the ancient Roman Empire, Pliny the Elder once said ‘On a piece of opal, you can see the beauty, colours and coherence of the ruby flame, amethyst stain and emerald sea’. A total of 95% of the world’s opals are mined in Australia. Opals with strong iridescent colours, large iridescent surface area and high transparency are considered top-grade opals (Fig. 5).
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