How to identify Black Tourmaline

To correctly identify Black Tourmaline (Schorl (Iron Tourmaline)), 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

   Typically opaque black, sometimes very dark blue or brownish-black.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Vitreous (glassy) to resinous.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Often found as elongated, prismatic crystals, sometimes radiating or massive. Crystals frequently exhibit prominent vertical striations on their prism faces.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Hexagonal-rhombohedral crystal system, typically forming elongated prismatic crystals with a characteristic triangular cross-section (though often appearing hexagonal due to prism faces). Terminations can be complex, often hemimorphic.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Indistinct to poor basal cleavage, often appearing to lack cleavage. Fracture is more common than cleavage.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Most commonly found in granitic pegmatites, high-temperature hydrothermal veins, and metamorphic rocks (schists, gneisses). Can also occur in granites and other igneous rocks.

Key Facts

Hardness

7-7.5 (Mohs scale)

Specific Gravity

3.06-3.26 g/cm³ (typically around 3.1-3.2)

Crystal System

Trigonal (hexagonal-rhombohedral)

Color

Opaque black, sometimes very dark blue or brownish-black

Luster

Vitreous to resinous

Transparency

Opaque

Fracture

Conchoidal to uneven

Cleavage

Indistinct to poor basal cleavage {0001}

Composition

Complex borosilicate of aluminum, iron, and other elements. End-member formula: NaFe₃Al₆(BO₃)₃Si₆O₁₈(OH)₄

Physical characteristics

• Crystal Habit: Prismatic, columnar, acicular, radiating, massive. Crystals often show prominent vertical striations on prism faces and a characteristic rounded triangular cross-section.
• Cleavage Type: Indistinct to poor basal cleavage {0001}. Typically fractures rather than cleaves.
• Fracture Type: Conchoidal to uneven.
• Tenacity: Brittle.
• Luster Type: Vitreous (glassy) to resinous.

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