physical properties of tourmalin

Density

2.82-3.32

Hardness

7-7.5

Crystal habit

Parallel and elongated. Acicular prisms, sometimes radiating. Massive. Scattered grains (in granite)

Crystal system

Trigonal

Cleavage

Indistinct

Fracture

Uneven, small conchoidal, brittle

Luster

Vitreous, sometimes resinous

Streak

White

Specific gravity

3.06 (+.20 -.06)

Polish luster

Vitreous

Optical properties

Double refractive, uniaxial negative

Refractive index

nω=1.635–1.675, nε=1.610–1.650

Birefringence

-0.018 to -0.040; typically about .020 but in dark stones it may reach .040

Pleochroism

typically moderate to strong

Red Tourmaline: Definite; dark red,light red
Green Tourmaline: Strong; dark green, yellow-green
Brown Tourmaline: Definite; dark brown, light brown
Blue Tourmaline: Strong; dark blue, light blue

Dispersion

.017

Ultraviolet

pink stones—inert to very weak

Fluorescence

red to violet in long and short wave

Absorption spectra

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