The facts
What sillimanite actually is
The species behind sillimanite is sillimanite, Al₂SiO₅; it forms in the orthorhombic system.
Fe³⁺ with Ti giving the violet-blue; greyish-green and colourless material also occurs is responsible for the colour — and the chromophore, not the name, is what decides whether a colour is stable.
On the instruments it reads a refractive index of 1.653–1.684 and a specific gravity of about 3.20–3.27, with perfect {010} cleavage.
Where sillimanite comes from
Commercial material comes from Ratnapura, Sri Lanka, Mogok, Myanmar, Idaho, USA and Orissa, India.
Different localities give different colour and different clarity, and the trade prices them differently. We state which one.
How sillimanite is treated
Split by material type. Transparent faceted sillimanite is essentially never treated and does not appear in the AGTA enhancement chart. Fibrous massive fibrolite, by contrast, is very often dyed and impregnated to imitate ruby, emerald and sapphire, and dyed fibrolite beads are openly traded out of India; the dye concentrates visibly in the fibrous structure and in surface-reaching fractures, and its stability is undocumented. Sources genuinely disagree on hardness (6 to 7.5 in reputable references), possibly because it is anisotropic — harder along the fibres than across.
We state the treatment on every stone, including when the honest answer is that it cannot be determined. A treatment disclosed is a fact; a treatment discovered later is a refund.
What sillimanite is confused with
Watch for Iolite, Kyanite, Tanzanite, Andalusite and Sapphire — these are what turn up sold under this name.
The separation is rarely a matter of opinion: refractive index 1.653–1.684 and specific gravity 3.20–3.27 settle most of it in a minute, and magnification settles the rest.
What sillimanite is used for
The trades that buy sillimanite are mineral collecting, jewellery and refractories. They do not want the same thing, and they do not pay in the same unit.
We quote each application in the unit that trade actually buys in, by the specimen, by the carat or by the tonne.
Durability and care
Hardness 6.5–7.5; cleavage perfect {010}. That combination decides what it can safely be set into.
Hardness is not the whole story. Perfect cleavage means the stone can split cleanly if struck along the wrong plane, whatever its Mohs number, so a protective setting matters more here than the hardness suggests.
Warm water, mild soap and a soft brush. Avoid ultrasonic cleaners on anything fracture-filled, stabilised, dyed or visibly included.
What sillimanite costs
Expect roughly $20–200/ct, depending on quality.
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 sillimanite treated, and how?
Split by material type. Transparent faceted sillimanite is essentially never treated and does not appear in the AGTA enhancement chart. Fibrous massive fibrolite, by contrast, is very often dyed and impregnated to imitate ruby, emerald and sapphire, and dyed fibrolite beads are openly traded out of India; the dye concentrates visibly in the fibrous structure and in surface-reaching fractures, and its stability is undocumented. Sources genuinely disagree on hardness (6 to 7.5 in reputable references), possibly because it is anisotropic — harder along the fibres than across.
Where does sillimanite come from?
Ratnapura, Sri Lanka, Mogok, Myanmar, Idaho, USA and Orissa, India. Origin affects price, so we name the locality rather than the country on every quote.
Is sillimanite hard enough to wear every day?
With care. It is 6.5–7.5 on the Mohs scale, which is hard enough, but perfect {010} cleavage means it can split if struck at the wrong angle. Use a protective setting and avoid ultrasonic cleaning.
How much does sillimanite cost?
Indicatively $20–200/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 sillimanite confused with?
Most often Iolite, Kyanite, Tanzanite, Andalusite and Sapphire. A refractive index of 1.653–1.684 and a specific gravity near 3.20–3.27 separate it quickly.
What is sillimanite used for besides jewellery?
mineral collecting and refractories. Each of those trades buys it in a different form and a different unit.
Related stones
Looking for sillimanite?
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 sillimanite