What it is confused with
Synthetic opal shows regular snakeskin or columnar patterns; doublets and triplets glue a thin slice of opal to a backing, visible as a straight join from the side.
White, black or transparent with spectral flashes
Opal crystals
Opal is hydrated silica (SiO₂·nH₂O) — 3 to 21 percent water by weight, white, black or transparent with spectral flashes, rated 5.5–6.5 on the Mohs scale. Traditionally, it is linked to imagination, emotional range and creative flow — attributed, not demonstrated.
- Mohs hardness
- 5.5–6.5 about as hard as window glass
- Crystal system
- Amorphous SiO₂·nH₂O
- Pliny, AD 77
- Book 37.21 as “opalus”
- Main sources today
- Australia also Ethiopia, Mexico
Pliny, Natural History, tr. Bostock and Riley.
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Properties
Composition
Hydrated silica (SiO₂·nH₂O) — 3 to 21 percent water by weight
Hardness (Mohs)
5.5–6.5
Crystal system
Amorphous
Color
White, black or transparent with spectral flashes
Mined in
Australia, Ethiopia, Mexico, Brazil
Worth knowing
The play of color comes from stacked silica spheres diffracting light — the same physics as a butterfly wing, not pigment.
History
Roman writers ranked it among the most valuable of gems; Australia has dominated supply since the 1890s.
Buying
Buying Opal: fakes, value and treatments
What decides the price
Brightness and pattern of the play of color and the darkness of the body. Black opal from Lightning Ridge in Australia is the most valuable.
Where it is used
Cabochons and inlay; boulder opal is cut with its ironstone host left on the back.
A less familiar fact
Australia supplies the great majority of the world's precious opal, and opal contains water — up to about a fifth of its weight.
Practical
Caring for Opal
How to clean it
Damp cloth only. Never ultrasonic, never steam, never leave it in a hot car — opal holds up to 21 percent water and crazes if it dries out.
How to tell it is real
Doublets and triplets — a thin opal slice glued to a dark backing — are common and much cheaper. Look at the stone edge-on: a doublet shows a flat glue line.
How hard is Opal?
Hardness is the lesser worry with opal. It holds up to 21 percent water, and heat or very dry air can make it craze with fine cracks — so no hot cars, steam or ultrasonic baths. Only Rhodonite shares its place on our scale. Among the stones in this reference, the next softer is Sodalite (5.5–6) and the next harder Moonstone (6–6.5).
Opal, at 5.5–6.5, is harder than 12 of the other 42 stones here and softer than 29.
Primary source
What Pliny the Elder wrote about Opal
“Of all precious stones, it is opal that presents the greatest difficulties of description, it displaying at once the piercing fire of carbunculus, the purple brilliancy of amethystos, and the sea-green of smaragdus, the whole blended together and refulgent with a brightness that is quite incredible.”
“On being thus proscribed, Nonius took to flight, carrying with him, out of all his wealth, nothing but this ring, the value of which, it is well known, was estimated at two millions of sesterces.”
Pliny the Elder, The Natural History, translated by John Bostock and H. T. Riley (London: H. G. Bohn, 1855–57), book 37, chapter 21 — read the original text.
Pliny’s opalus is the opal we know. The senator Nonius was proscribed by Mark Antony for the sake of his opal and fled Rome with nothing but the ring. Who Pliny was and why we quote him →
Traditional associations
Opal is traditionally linked to imagination, emotional range and creative flow. Chakra in crystal practice: Crown, Heart. Element: Water.
Questions about Opal
What is Opal made of?
Hydrated silica (SiO₂·nH₂O) — 3 to 21 percent water by weight, with no crystal structure (amorphous).
Which month is Opal the birthstone for?
October.
Where does Opal come from?
Australia, Ethiopia, Mexico, Brazil.
Can Opal be worn every day?
With care. At 5.5–6.5 on the Mohs scale it can be worn often, but watch out for heat and dry air, which make it craze.
Sources
Statements of fact on this page were checked against these sources in October 2026. How we choose and check sources: editorial policy.
Cite this page
AstroHot. “Opal Crystals: Meaning, Hardness and Origins.” Accessed October 28, 2026. https://astrohot.com/crystals/gemstones/opal/
For Wikipedia
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