The facts
What blue John actually is
Blue John is classified as fluorite (banded variety), with the composition CaF₂. It crystallises in the cubic (banded aggregate) system.
Colour comes from purple-blue banding from colour centres associated with included hydrocarbons and natural irradiation, which is also what a laboratory looks at when it is asked whether a colour is natural.
Refractive index 1.434. Specific gravity around 3.0–3.2. Cleavage: perfect octahedral, which is why it must be resin-stabilised before turning. Those three numbers separate it from most of what imitates it.
Where blue John comes from
Blue John is produced at Treak Cliff Cavern, Castleton, Derbyshire, England and Blue John Cavern, Castleton, Derbyshire, England.
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 blue John is treated
Blue John is always stabilised: traditionally boiled in pine resin, now vacuum-impregnated with epoxy, because the cleavage makes untreated material unturnable. This is standard practice and not a deception. Supply is tiny — extraction is limited to a few tonnes a year from two caverns — and imitations in dyed fluorite and resin exist.
Treatment changes value, sometimes by a multiple, so it belongs in the description before the price and not after the sale.
What blue John is confused with
In the trade blue John is most often mistaken for, or substituted by, Other banded fluorite, Dyed fluorite, Resin imitations or Amethyst.
The separation is rarely a matter of opinion: refractive index 1.434 and specific gravity 3.0–3.2 settle most of it in a minute, and magnification settles the rest.
What blue John is used for
Demand for blue John comes from decorative objects, carving and sculpture, jewellery, mineral collecting and architecture and interiors, which is why one price per carat never covers it.
We quote each application in the unit that trade actually buys in, by the piece, by the block or by the carat.
Durability and care
Blue John is 4 on the Mohs scale, with perfect octahedral, which is why it must be resin-stabilised before turning 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 blue John costs
As a guide, blue John trades around Small jewellery pieces $30–$300; turned bowls and vases $500–$10,000; antique Georgian urns far higher.
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 blue John treated, and how?
Blue John is always stabilised: traditionally boiled in pine resin, now vacuum-impregnated with epoxy, because the cleavage makes untreated material unturnable. This is standard practice and not a deception. Supply is tiny — extraction is limited to a few tonnes a year from two caverns — and imitations in dyed fluorite and resin exist.
Where does blue John come from?
Treak Cliff Cavern, Castleton, Derbyshire, England and Blue John Cavern, Castleton, Derbyshire, England. Origin affects price, so we name the locality rather than the country on every quote.
Is blue John hard enough to wear every day?
Not really. At 4 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 blue John cost?
Indicatively Small jewellery pieces $30–$300; turned bowls and vases $500–$10,000; antique Georgian urns far higher. 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 blue John confused with?
Most often Other banded fluorite, Dyed fluorite, Resin imitations and Amethyst. A refractive index of 1.434 and a specific gravity near 3.0–3.2 separate it quickly.
What is blue John used for besides jewellery?
decorative objects, carving and sculpture, mineral collecting and architecture and interiors. Each of those trades buys it in a different form and a different unit.
Related stones
Looking for blue john?
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 blue john