mineral

Black tourmaline (schorl), possibly in matrix

Schorl tourmaline — NaFe2+3Al6(BO3)3Si6O18(OH)4

AI-generated identification · may be incorrect

Dark black, vitreous-to-submetallic luster with strongly striated prismatic faces and uneven fracture; Mohs hardness 7–7.5, specific gravity about 3.0–3.3. The apparent columnar form and longitudinal grooves are characteristic, though the specimen is partly rough or matrix-attached.

Identified August 19, 2026More mineral →

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Physical properties

Formation & geological history

Forms in boron-rich granitic pegmatites and hydrothermal veins during late-stage crystallization, commonly in Precambrian to younger continental crust. The São Paulo region lies within the Borborema–Ribeira-related crystalline terrains and has plausible pegmatitic tourmaline, although image-only identification is approximate.

Uses & applications

Collected as a common black crystal, used in mineral specimens and occasionally ornamental stone; gem-quality transparent tourmaline is cut for jewelry, but this opaque piece is mainly a collector specimen.

Geological facts

Tourmaline is strongly piezoelectric and can become electrically charged when heated or rubbed. Schorl is the most common tourmaline species and is usually iron-rich and black.

Field identification & locations

Look for a six-sided prismatic crystal with deep lengthwise striations, no obvious cleavage, and attraction to static effects rather than a magnet. Similar black minerals include hornblende, biotite, and ilmenite; hardness and crystal shape help distinguish them.