Crystal Black Tourmaline

Chemical Formula

The simple chemical formula, which covers the main forms of Tourmaline (Elbaite, Schorl, and Dravite), is as follows:
(Na,Ca)(Mg,Li,Al,Fe²⁺)₃Al₆(BO₃)₃Si₆O₁₈(OH)₄

The expanded formula, which additionally covers Uvite, Liddicoatite, and Buergerite, is as follows:
(Na,Ca)(Mg,Li,Al,Fe²⁺,Fe³⁺)₃(Al,Mg)₆(BO₃)₃Si₆O₁₈(OH,O,F)₄

The formula for the Tourmaline group is very complex. See The chemical formula of Tourmaline for more details.
 

Composition

See The chemical formula of Tourmaline.

Color

Tourmaline is extremely varied in color. Colors include black, brown, green, red, pink, blue, and gray. White, colorless, yellow, orange, and purple colors are less common. Crystals are frequently multicolored, containing two or more distinct colors. Some specimens are pleochroic.

Streak

Whie

Hardness

7 - 7.5

Crystal System

Hexagonal

Crystal Forms 
and Aggregates

Usually as elongated prismatic crystals that are heavily striated. Also as short, stubby, prismatic crystals. Most Tourmaline crystals have a rounded, triangular cross-section. Seldom in tabular crystals. Aggregates include columnar, radiating, botryoidal, stalactitic, in dense groups of tiny, elongated needles, and in compact masses.

Transparency

Transparent to opaque

Specific Gravity

2.9 - 3.3

Luster

Vitreous. Some black and brown specimens may be dull.

Cleavage

3,2

Fracture

Conchoidal to uneven

Tenacity

Brittle

Other ID Marks

1) Strongly pyroelectric.
2) Piezoelectric.
3) A few forms of Tourmaline fluoresce yellow in shortwave ultraviolet light.
 

In Group

Silicates; Cyclosilicates; Tourmaline Group

Striking Features

Color, crystal form, hardness, and deep vertical striations.

Environment

Elbaite, Schorl, and Liddicoatite are almost exclusively from granite pegmatites, while Dravite and Uviteor mostly from metamorphic environments such as marbles. Buergerite is from igneous rhyolite deposits.
 

Rock Type

Igneous, Metamorphic

Popularity (1-4)

1

Prevalence (1-3)

2

Demand (1-3)

1