Black Tourmaline

Description

Schorl is the most common member of the tourmaline group, accounting for approximately 95% of all natural tourmaline. It is an iron-rich, black, opaque borosilicate mineral. Its characteristic prismatic crystals, often with vertical striations, are easily recognizable. It is known for its pyroelectric and piezoelectric properties, meaning it can generate an electrical charge when heated/cooled or subjected to mechanical stress. Schorl is a complex borosilicate with a variable chemical composition, but its defining characteristic is a high iron content, which gives it its distinctive black color.

How to Identify

Color

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

Luster

Vitreous (glassy) to resinous.

Texture

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

Crystal Form

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.

Cleavage

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

Geological Environment

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

Quick Check

• Color: Opaque black
• Luster: Vitreous to resinous
• Streak: White to colorless

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.

Formation

Schorl forms primarily in granitic pegmatites, high-temperature hydrothermal veins, and metamorphic rocks such as schists and gneisses. It crystallizes from boron-rich, silica-saturated magmatic or hydrothermal fluids. In pegmatites, it often forms large, well-developed crystals. In metamorphic environments, it can be a product of metasomatism.

Usage

Historically, schorl was used as a mourning stone. Today, it is widely used as a gemstone (though typically opaque and not faceted), in metaphysical practices for protection and grounding, and as a collector's mineral. Due to its pyroelectric and piezoelectric properties, it has niche applications in pressure gauges and other electronic devices, though less commonly than other tourmaline varieties or quartz.

Age Distribution

Found in rocks of various ages, from Precambrian to Cenozoic, depending on the geological setting.

Where to Find

Finding Tips

Similar Rocks

Scientific Classification

Mineral Class

Silicate (Cyclosilicate)

Group

Tourmaline Group

Crystal System

Trigonal

Chemical Formula

NaFe₃Al₆(BO₃)₃Si₆O₁₈(OH)₄ (idealized end-member formula)

Composition

Sodium iron aluminum borosilicate hydroxide. Contains significant amounts of iron (Fe), which gives it its black color. Other elements like magnesium (Mg) and lithium (Li) are present in other tourmaline varieties but are minor or absent in pure schorl.

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