The facts
What hardystonite actually is
Hardystonite is classified as melilite group, with the composition Ca₂ZnSi₂O₇. It crystallises in the tetragonal system.
Colour comes from colourless to pale pink-brown in daylight; its fame is the intense violet-blue short-wave fluorescence, which is also what a laboratory looks at when it is asked whether a colour is natural.
Refractive index 1.669–1.675. Specific gravity around 3.40–3.44. Cleavage: distinct basal. Those three numbers separate it from most of what imitates it.
Where hardystonite comes from
Hardystonite is produced at Franklin Mine, Sussex County, New Jersey, USA.
That is the whole list. A single source makes supply fragile and makes any claim of a second origin worth questioning.
How hardystonite is treated
Untreated. Cut as fluorescent cabochons and display pieces; too soft for wear. The fluorescent response is intrinsic, not an added coating.
Treatment changes value, sometimes by a multiple, so it belongs in the description before the price and not after the sale.
What hardystonite is confused with
In the trade hardystonite is most often mistaken for, or substituted by, Calcite, Willemite or Clinohedrite.
The separation is rarely a matter of opinion: refractive index 1.669–1.675 and specific gravity 3.40–3.44 settle most of it in a minute, and magnification settles the rest.
What hardystonite is used for
Demand for hardystonite comes from mineral collecting and decorative objects, which is why one price per carat never covers it.
We quote each application in the unit that trade actually buys in, by the specimen or by the piece.
Durability and care
Hardystonite is 3–4 on the Mohs scale, with distinct basal 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 hardystonite costs
As a guide, hardystonite trades around Fluorescent specimens 30–400 USD.
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 hardystonite treated, and how?
Untreated. Cut as fluorescent cabochons and display pieces; too soft for wear. The fluorescent response is intrinsic, not an added coating.
Where does hardystonite come from?
Franklin Mine, Sussex County, New Jersey, USA. That is the only commercial source.
Is hardystonite hard enough to wear every day?
Not really. At 3–4 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 hardystonite cost?
Indicatively Fluorescent specimens 30–400 USD. 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 hardystonite confused with?
Most often Calcite, Willemite and Clinohedrite. A refractive index of 1.669–1.675 and a specific gravity near 3.40–3.44 separate it quickly.
Related stones
Looking for hardystonite?
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 hardystonite