oxides
Opal
SiO2·nH2O
- Also known as
- precious opalcommon opalpotch
- Crystal system
- amorphous
- Hardness
- 5.5–6.5 on the Mohs scale
- Lustre
- vitreous
- Colour
- White, black, orange or colourless, often with spectral play of colour
- Where it forms
- From silica-bearing groundwater percolating through rock at low temperature and setting as a gel in cavities and cracks. Australian fields formed in deeply weathered Cretaceous sediments over millions of years.
- Chiefly from
- Coober Pedy and Lightning Ridge, AustraliaWollo, EthiopiaQuerétaro, MexicoBrazil
Why it is that colour
The flashes are not pigment. Precious opal is built of silica spheres packed in a regular array, and where the spheres are close to the wavelength of visible light they diffract it — the same physics as a butterfly wing or an oil film, not a colouring agent.
Safety
Safe to handle. It contains water and can craze or crack if allowed to dry out or if subjected to sudden temperature change — a conservation issue rather than a health one.
Where it comes from
Australia has historically supplied the majority of precious opal, from small-scale claims at Coober Pedy and Lightning Ridge. Ethiopian material from Wollo, found in 2008, now takes a large share; it is more porous, more prone to absorbing liquid and changing appearance, and much of it is treated.
Historical use
A prized Roman gem — the story of the senator Nonius, who supposedly chose exile over surrendering his opal to Mark Antony, is told by Pliny. Later a mainstay of Art Nouveau jewellery, and Australia's national gemstone.
Recorded as history and attributed where possible. That a stone was used for something establishes that people used it, not that it worked.
What is attributed to it, and since when
Highly regarded in the classical world; Pliny writes about it admiringly. The reputation for bad luck is much later and its most-cited cause is literary: Walter Scott's 1829 novel Anne of Geierstein, in which an opal loses its fire and its owner dies. The European opal market is said to have fallen sharply afterwards. Whether the novel truly caused it or merely fixed an existing superstition, the modern trade still deals with the belief.
Described, not endorsed. Dating a claim is not the same as assessing it — but an association that first appears in a book published in 1985 is not an ancient inheritance, whatever the label says. A sincere account of feeling better is compatible with the stone doing nothing — see the placebo effect.
Sold as this, but not
Worth knowing
Opal is a genuine oddity in the IMA list. A mineral is supposed to have an ordered crystal structure, and opal does not — it is amorphous hydrated silica. It sits on the list as a grandfathered species, accepted because it was long established before the modern definition was written. Strictly, it is a mineraloid.
The play of colour was not understood until the 1960s, when electron microscopy showed the packed silica spheres. Before that, nobody could explain why some opal flashed and some did not; the answer is simply whether the spheres are uniform enough and the right size to diffract visible light. Common opal, or potch, has the same chemistry and disordered spheres, and that is the entire difference between a stone worth thousands and one worth nothing.
Identity checked against IMA-CNMNC master list, June 2025.