The facts
What boulder Opal actually is
Mineralogically, boulder Opal is opal in ironstone matrix — SiO₂·nH₂O on Fe-rich matrix, crystallising in the amorphous system.
The colour is caused by play-of-colour by diffraction, seen against a natural dark ironstone backing. That is worth knowing because it determines what heating, light and time can and cannot do to the stone.
A refractometer reads 1.44–1.47, and the specific gravity sits near 2.40–3.00 (with matrix); cleavage is none.
Where boulder Opal comes from
The recognised sources are Quilpie, Queensland, Australia, Yowah, Queensland, Australia and Koroit, Queensland, Australia.
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 boulder Opal is treated
Untreated — the dark backing is natural host rock still attached, which is what distinguishes a genuine boulder opal from a man-made doublet. SG readings are meaningless because they average opal and ironstone.
None of this is a problem when it is said out loud. It becomes one the moment it is left out.
What boulder Opal is confused with
The usual confusions are Opal doublets, Matrix opal and Fire agate.
The separation is rarely a matter of opinion: refractive index 1.44–1.47 and specific gravity 2.40–3.00 (with matrix) settle most of it in a minute, and magnification settles the rest.
What boulder Opal is used for
Boulder Opal is bought by jewellery, mineral collecting and decorative objects — 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, by the specimen or by the piece.
Durability and care
Boulder 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 boulder Opal costs
Indicative pricing for boulder Opal is $20–2,000/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 boulder Opal treated, and how?
Untreated — the dark backing is natural host rock still attached, which is what distinguishes a genuine boulder opal from a man-made doublet. SG readings are meaningless because they average opal and ironstone.
Where does boulder Opal come from?
Quilpie, Queensland, Australia, Yowah, Queensland, Australia and Koroit, Queensland, Australia. Origin affects price, so we name the locality rather than the country on every quote.
Is boulder 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 boulder Opal cost?
Indicatively $20–2,000/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 boulder Opal confused with?
Most often Opal doublets, Matrix opal and Fire agate. A refractive index of 1.44–1.47 and a specific gravity near 2.40–3.00 (with matrix) separate it quickly.
What is boulder Opal used for besides jewellery?
mineral collecting and decorative objects. Each of those trades buys it in a different form and a different unit.
Related stones
Looking for boulder 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 boulder opal