The facts
What opal actually is
Mineralogically, opal is hydrated silica — SiO₂·nH₂O, crystallising in the amorphous system.
The colour is caused by diffraction from an ordered array of silica spheres. That is worth knowing because it determines what heating, light and time can and cannot do to the stone.
A refractometer reads 1.37–1.52, and the specific gravity sits near 1.98–2.25; cleavage is none.
Where opal comes from
The recognised sources are Coober Pedy and Lightning Ridge, Australia, Wollo, Ethiopia, Querétaro, Mexico and Brazil.
Origin affects price in this material, so we name the locality on every quote rather than the country. "African" or "Asian" tells a buyer nothing.
How opal is treated
Ethiopian opal is porous and can be dyed or smoke-treated. Doublets and triplets are assembled, not treated.
None of this is a problem when it is said out loud. It becomes one the moment it is left out.
What opal is confused with
The usual confusions are Synthetic opal, Opalite glass and Doublets and triplets.
The separation is rarely a matter of opinion: refractive index 1.37–1.52 and specific gravity 1.98–2.25 settle most of it in a minute, and magnification settles the rest.
What opal is used for
Opal is bought by jewellery and mineral collecting — and each of those trades wants a different form of the same material.
We quote each application in the unit that trade actually buys in, by the carat or by the specimen.
Durability and care
Opal is 5.5–6.5 on the Mohs scale, with none cleavage.
With no cleavage plane to worry about, the practical limit is abrasion rather than breakage, and it takes daily wear well.
Warm water, mild soap and a soft brush. Avoid ultrasonic cleaners on anything fracture-filled, stabilised, dyed or visibly included.
What opal costs
Indicative pricing for opal is $10–10,000/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 opal treated, and how?
Ethiopian opal is porous and can be dyed or smoke-treated. Doublets and triplets are assembled, not treated.
Where does opal come from?
Coober Pedy and Lightning Ridge, Australia, Wollo, Ethiopia, Querétaro, Mexico and Brazil. Origin affects price, so we name the locality rather than the country on every quote.
Is opal hard enough to wear every day?
Yes. At 5.5–6.5 on the Mohs scale with no cleavage plane, it handles daily wear well. The practical limit is gradual abrasion rather than chipping.
How much does opal cost?
Indicatively $10–10,000/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 opal confused with?
Most often Synthetic opal, Opalite glass and Doublets and triplets. A refractive index of 1.37–1.52 and a specific gravity near 1.98–2.25 separate it quickly.
Related stones
Looking for 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 opal