How to identify Black Tourmaline

To correctly identify Black Tourmaline (Schorl (NaFe3Al6(BO3)3Si6O18(OH)4)), check each of these features in order. Many rocks and minerals look alike, so cross-reference multiple properties before deciding.

Step-by-step identification

1. 1

   Color

   Overall color and any zoning or banding

   Opaque, jet-black to very dark brown. Tumbled pieces will retain this color.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Vitreous to resinous, sometimes sub-metallic on fresh surfaces. Tumbled surfaces enhance the vitreous luster.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Smooth and polished due to tumbling. Untumbled specimens often show striations parallel to the c-axis.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Though tumbled, original forms are typically elongated prismatic crystals with characteristic triangular cross-sections and vertical striations. Tumbled pieces will be rounded.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Poor to indistinct, often appearing as conchoidal fracture rather than true cleavage.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Granitic pegmatites, high-temperature hydrothermal veins, and metamorphic rocks (schists, gneisses).

Key Facts

Hardness

7-7.5 (Mohs scale)

Specific Gravity

3.1-3.26 g/cm³

Crystal System

Trigonal

Color

Opaque black

Luster

Vitreous to resinous

Transparency

Opaque

Fracture

Conchoidal to uneven

Cleavage

Poor to indistinct (basal)

Composition

Sodium iron aluminum borosilicate hydroxide

Physical characteristics

• Crystal Habit: Typically prismatic, often with vertical striations and a characteristic triangular cross-section. Tumbled forms are rounded and polished.
• Cleavage Type: Poor to indistinct, often parting rather than true cleavage, typically basal.
• Fracture Type: Conchoidal to uneven, brittle.
• Tenacity: Brittle
• Luster Type: Vitreous to resinous, sometimes sub-metallic

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