tourmalines

Key facts

Mineral.color

Most commonly black, but can range from colorless to brown, red, orange, yellow, green, blue, violet, pink, or hues in-between. It can also be bi-colored, or even tri-colored. Rarely, it can be found as neon green or electric blue.

Mineral.habit

Parallel and elongated; acicular prisms, sometimes radiating; massive; scattered grains (in granite)

Mineral.cleavage

Indistinct

Mineral.fracture

Uneven, small conchoidal

Mineral.tenacity

Brittle

Mineral.mohs

7.0–7.5

Mineral.luster

Vitreous, sometimes resinous

Mineral.polish

Vitreous

Mineral.refractive

n = 1.635–1.675n = 1.610–1.650

Mineral.opticalprop

Double-refractive, uniaxial negative

Mineral.birefringence

−0.018 to −0.040; typically about −0.020 but in dark stones it may reach −0.040

Mineral.dispersion

0.017

Mineral.fluorescence

Pink stones; inert to very weak red to violet in long and short wave

Mineral.absorption

Strong narrow band at 498 nm, and almost complete absorption of red down to 640 nm in blue and green stones; red and pink stones show lines at 458 and 451 nm, as well as a broad band in the green spectrum

Mineral.streak

White

Mineral.gravity

3.06+0.20–0.06

Mineral.density

2.82–3.32

Mineral.diaphaneity

Translucent to opaque

via Wikipedia infobox