The facts
What common Opal actually is
Common Opal is classified as opal, with the composition SiO₂·nH₂O. It crystallises in the amorphous system.
Colour comes from no play-of-colour; body colours from Cu (blue-green Peruvian), Fe, Mn dendrites or organic matter, which is also what a laboratory looks at when it is asked whether a colour is natural.
Refractive index 1.42–1.47. Specific gravity around 1.90–2.20. Cleavage: none. Those three numbers separate it from most of what imitates it.
Where common Opal comes from
Common Opal is produced at Acarí, Arequipa, Peru, Quincy, France, Mapimí, Durango, Mexico and Bela Vista, Brazil.
The locality is part of the description, not a footnote — two stones of the same weight from two of these places are not the same stone.
How common Opal is treated
Generally untreated; porous grades are sometimes resin-stabilised or dyed. Pink and blue Peruvian opals may be lightly dyed to even out colour.
Treatment changes value, sometimes by a multiple, so it belongs in the description before the price and not after the sale.
What common Opal is confused with
In the trade common Opal is most often mistaken for, or substituted by, Chrysocolla, Chalcedony, Dyed opal or Plastic.
The separation is rarely a matter of opinion: refractive index 1.42–1.47 and specific gravity 1.90–2.20 settle most of it in a minute, and magnification settles the rest.
What common Opal is used for
Demand for common Opal comes from jewellery, carving and sculpture, decorative objects, architecture and interiors and mineral collecting, which is why one price per carat never covers it.
We quote each application in the unit that trade actually buys in, by the carat, by the block or by the piece.
Durability and care
Common Opal is 5–6.5 on the Mohs scale, with none cleavage.
Below about 5.5 a stone will be scratched by ordinary household dust, which is largely quartz. That does not make it unusable — it makes it a pendant, an earring or an object rather than a ring worn every day.
Warm water, mild soap and a soft brush. Avoid ultrasonic cleaners on anything fracture-filled, stabilised, dyed or visibly included.
What common Opal costs
As a guide, common Opal trades around $1–40/ct.
That is a range, not a quote. Colour, size, clarity, cut and origin move the figure a long way inside it, and the only honest price is one given against a specific stone. Send us what you need and we will price it.
Questions
Is common Opal treated, and how?
Generally untreated; porous grades are sometimes resin-stabilised or dyed. Pink and blue Peruvian opals may be lightly dyed to even out colour.
Where does common Opal come from?
Acarí, Arequipa, Peru, Quincy, France, Mapimí, Durango, Mexico and Bela Vista, Brazil. Origin affects price, so we name the locality rather than the country on every quote.
Is common Opal hard enough to wear every day?
Not really. At 5–6.5 on the Mohs scale it is softer than the quartz dust in ordinary household dirt, so it scratches with wear. It suits pendants, earrings and objects better than a daily ring.
How much does common Opal cost?
Indicatively $1–40/ct. Colour, size, clarity and origin move the price a long way within that, so treat it as a range and ask for a quote against a specific stone.
What is common Opal confused with?
Most often Chrysocolla, Chalcedony, Dyed opal and Plastic. A refractive index of 1.42–1.47 and a specific gravity near 1.90–2.20 separate it quickly.
What is common Opal used for besides jewellery?
carving and sculpture, decorative objects, architecture and interiors and mineral collecting. Each of those trades buys it in a different form and a different unit.
Related stones
Looking for common opal?
Tell us the size, the quality and what it is for. We source it, photograph it and quote it — with the locality named.
Ask about common opal