Black Tourmaline

Black tourmaline - properly called schorl - is the most common variety of tourmaline, making up roughly 95% of all tourmaline found in nature. It forms in granitic pegmatites and metamorphic rocks where boron-rich fluids interact with iron-bearing minerals.

The deep black color comes from high iron content in the crystal structure. Schorl crystallizes as elongated, striated prisms with a distinctive rounded triangular cross-section - a shape unique to the tourmaline group. The striations (parallel grooves running lengthwise on the crystal faces) are caused by oscillating growth between different crystal faces during formation.

Black tourmaline is both pyroelectric and piezoelectric - it generates electrical charges when heated or compressed. This property was observed centuries before it was scientifically explained, contributing to its reputation as a protective stone across many cultures.

Black tourmaline is identified by its characteristic striated prismatic crystals with a rounded triangular cross-section. No other common black mineral shows this combination of features. At hardness 7, it's harder than most other black minerals (obsidian is 5.5, hornblende is 5-6).

Distinguish from obsidian (glassy, no crystal faces), black hornblende (perfect cleavage in two directions), and black augite (stubby crystals, not elongated). The striations are the quickest diagnostic - if a black crystal has prominent parallel grooves running its length, it's almost certainly tourmaline.