Properties

The measurements, and what they are good for

Tourmaline reads between 1.619 and 1.655 on a refractometer, with a birefringence of roughly 0.018 to 0.020 and a specific gravity around 3.06. Those three numbers identify the group with near certainty — and tell you nothing at all about which tourmaline you are holding.

Mineral groupTourmaline — a group of more than thirty species, not a single mineral

Chemical classCyclosilicate with borate groups (borosilicate)

Crystal systemTrigonal, space group R3m

HabitElongate prisms striated parallel to the c-axis; rounded triangular cross-section; hemimorphic, with different terminations at each end

Mohs hardness7 – 7.5

Specific gravity3.03 – 3.25 · elbaite around 3.06, schorl and dravite higher

Refractive index1.619 – 1.655

Birefringence0.018 – 0.020, higher in iron-rich material

Optic characterUniaxial negative

Dispersion0.017 — low; tourmaline shows little fire, and reads on colour instead

PleochroismModerate to strong dichroism; the c-axis direction is always the darker one

CleavageNone to indistinct

FractureConchoidal to uneven; brittle in heavily included material

LustreVitreous

FluorescenceGenerally inert; some pink material shows a weak reaction

OtherPiezoelectric and pyroelectric — develops a charge under pressure or a change in temperature

In practice

Four things the table does not say

The instruments will not name the variety

Every gem tourmaline reads within the same narrow range, so a refractometer separates tourmaline from apatite, zircon or glass, but never a rubellite from a Paraíba. Variety and origin come from chemistry, not from optics.

Apatite is the useful comparison

Neon blue apatite looks strikingly like cuprian tourmaline, but its birefringence is 0.002–0.004 against tourmaline's 0.018–0.020, its hardness is 5, and its specific gravity is 3.18–3.22. Ten seconds on a refractometer ends the question.

Pleochroism decides how the stone is cut

Because the c-axis direction is darker, a cutter who orients the table perpendicular to it gets a light, open stone; one who does not gets a closed dark one. In dark material the orientation is worth more than the weight.

Brittleness, not softness, is the risk

At 7–7.5 tourmaline resists scratching well. What it does not resist is a knock on a stone already carrying feathers, or thermal shock. Included material and heavily zoned crystals are the vulnerable ones.

Care

How to clean it, and what will damage it

Warm water, mild soap, a soft brush. That is the whole method.
No ultrasonic and no steam on included, fractured or heavily zoned material — the risk is a feather opening, not a scratch.
Avoid sudden temperature change. Tourmaline is pyroelectric: heat it and it attracts dust to itself, which is a nuisance, and shocks it, which is worse.
Keep prolonged strong light off pink and red material, and store stones separately — 7.5 will scratch 7.5.

The chemistry behind these numbers is set out on the chemistry page, and what the colours mean on the colour range.