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Tumbled Black Tourmaline, or Schorl, is the most common member of the complex borosilicate mineral group known as tourmaline. It is characterized by its opaque, jet-black color and often exhibits a vitreous to resinous luster. When tumbled, its natural prismatic crystal habit is smoothed, resulting in rounded, polished forms. Schorl is known for its strong pleochroism (though less apparent in opaque black varieties) and its pyroelectric and piezoelectric properties, meaning it can generate an electrical charge when heated or subjected to pressure. Its chemical formula, NaFe3Al6(BO3)3Si6O18(OH)4, highlights its sodium, iron, aluminum, boron, silicon, oxygen, and hydroxyl content, with iron being responsible for its characteristic black color.
How to Identify
- Color
- Opaque, jet-black to very dark brown. Tumbled pieces will retain this color.
- Luster
- Vitreous to resinous, sometimes sub-metallic on fresh surfaces. Tumbled surfaces enhance the vitreous luster.
- Texture
- Smooth and polished due to tumbling. Untumbled specimens often show striations parallel to the c-axis.
- Crystal Form
- Though tumbled, original forms are typically elongated prismatic crystals with characteristic triangular cross-sections and vertical striations. Tumbled pieces will be rounded.
- Cleavage
- Poor to indistinct, often appearing as conchoidal fracture rather than true cleavage.
- Geological Environment
- 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
Quick Check
- Color: Opaque, jet-black
- Luster: Vitreous to resinous
- Streak: White to colorless
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
Formation
Schorl typically forms in granitic pegmatites, high-temperature hydrothermal veins, and metamorphic rocks such as schists and gneisses. It crystallizes from boron-rich fluids during the final stages of magma crystallization or during regional and contact metamorphism. The presence of iron (Fe) is crucial for its black coloration.
Usage
Historically, tourmaline was used as a gemstone and for its pyroelectric and piezoelectric properties. Black tourmaline (schorl) is widely used in jewelry, as an ornamental stone, and in metaphysical practices. Industrially, tourmaline has limited use, primarily in pressure gauges due to its piezoelectricity, though synthetic alternatives are more common. Tumbled black tourmaline is popular for decorative purposes, crystal healing, and as a collector's item.
Age Distribution
Formed throughout various geological eras, often associated with late-stage magmatic and metamorphic processes.
Where to Find
Brazil
Minas Gerais is a significant source of various tourmaline varieties, including schorl, often found in large pegmatite deposits.
Pakistan
Known for high-quality tourmaline, including schorl, from pegmatite occurrences in the northern regions.
Afghanistan
Similar to Pakistan, pegmatite deposits yield various tourmalines, including schorl.
United States
Localities in Maine, California, and Colorado have produced schorl, particularly from pegmatite intrusions.
Africa (e.g., Namibia, Mozambique, Madagascar)
Numerous countries in Africa are rich in pegmatite fields, yielding abundant schorl.
Finding Tips
Geological Context
Look for schorl in areas known for granitic pegmatites, which are coarse-grained igneous rocks, or in metamorphic terrains where boron-rich fluids were active.
Associated Minerals
Schorl often occurs with quartz, feldspar (orthoclase, albite), mica (muscovite, biotite), beryl, and other pegmatite minerals.
Crystal Habit
Even in tumbled form, the density and characteristic black color are key. Untumbled specimens will show the distinctive prismatic crystals with vertical striations and a triangular cross-section.
Hardness Test
Schorl is relatively hard (7-7.5 on Mohs scale), so it will scratch glass and will not be easily scratched by a steel knife.
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Scientific Classification
- Mineral Class
- Silicate (Cyclosilicate)
- Group
- Tourmaline Group
- Crystal System
- Trigonal
- Chemical Formula
- NaFe3Al6(BO3)3Si6O18(OH)4
- Composition
- Sodium iron aluminum borosilicate hydroxide
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