Coticule

What coticule actually is

The species behind coticule is a metamorphic rock in which 5-20 micrometre spessartine garnet crystals are dispersed through a mica-rich phyllite matrix - a natural garnet abrasive in a self-releasing binder, Spessartine Mn3Al2(SiO4)3 approx. 30-42%, in muscovite, paragonite and quartz; it forms in the aggregate (cubic garnet in a phyllosilicate matrix) system.

Pale yellow-cream from the matrix; the blue backing layer is a distinct manganese-bearing slate 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 1.79-1.81 (spessartine) and a specific gravity of about 2.70-2.90, with none in the garnet; the matrix is micaceous cleavage.

Where coticule comes from

Commercial material comes from Vielsalm, Salm-Chateau and Lierneux, Ardennes, Liege Province, Belgium - a single small district and the only true source and Regne and Thier del Preu veins, Ardennes, Belgium.

Different localities give different colour and different clarity, and the trade prices them differently. We state which one.

How coticule is treated

Untreated; traditionally bonded to a blue slate ('la Belge' or Belgian Blue) backing, which is both structural and a coarser secondary hone. Only a handful of small operations still quarry it, and the classic named veins (La Dressante, La Veinette, Les Latneuses) command different prices for genuine technical reasons - the garnet size and density vary by vein. Synthetic 'coticule-style' stones and unrelated yellow slates are sold under the name. Not CITES.

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 coticule is confused with

Watch for Belgian Blue whetstone (the backing layer, a different and coarser stone), Synthetic garnet-loaded stones and Yellow slates and siltstones sold as coticule — these are what turn up sold under this name.

The separation is rarely a matter of opinion: refractive index 1.79-1.81 (spessartine) and specific gravity 2.70-2.90 settle most of it in a minute, and magnification settles the rest.

What coticule is used for

The trades that buy coticule are sharpening stones, abrasives and mineral collecting. 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 stone, by the tonne or by the specimen.

Durability and care

Coticule is Garnet 7-7.5 in a matrix around 3-4 - the softness of the binder is what makes it work on the Mohs scale, with none in the garnet; the matrix is micaceous 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 coticule costs

Expect roughly $80-350 for a small bout or rubbing stone; $200-900 for a full-size vein-identified bench coticule, 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.

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