Dravite

Dravite is the magnesium-rich brown variety of tourmaline (NaMg₃Al₆(BO₃)₃Si₆O₁₈(OH)₄), one of the recognized species within the tourmaline group of boron silicate minerals. Dravite crystallizes primarily in metamorphic environments where boron-bearing fluids interact with magnesium-rich rocks. Unlike its lithium-rich cousin elbaite, which forms in pegmatites, dravite favors contact metamorphic zones where granitic intrusions bake adjacent magnesian limestones, dolomites, and mafic rocks. Temperatures between 400°C and 650°C and moderate pressures drive the reaction of boron-rich fluids with available magnesium, aluminum, and silicon to build dravite's complex crystal structure.

The magnesium content is what distinguishes dravite from other tourmaline species. In the tourmaline general formula, the Y-site accommodates three cations, and in dravite these are predominantly magnesium ions. This magnesium dominance produces the characteristic brown coloration, though the exact shade varies depending on iron content and the ratio of Mg to Fe. Specimens with higher iron develop darker brown to near-black colors, while purer magnesium endmembers can appear golden or honey-toned.

Dravite also occurs in some boron-rich granitic pegmatites and in evaporite-associated metamorphic terrains. In Western Australia, large dravite crystals grow in metamorphosed sediments that were originally boron-rich marine deposits. The boron was concentrated by evaporation in shallow basins millions of years before metamorphism transformed the sediments and mobilized the boron into tourmaline.

Dravite shows the classic tourmaline trigonal prismatic habit with a rounded triangular cross-section and strong vertical striations. Brown to dark brown color immediately distinguishes it from most other tourmaline species. Hardness of 7 to 7.5, vitreous to slightly resinous luster, and conchoidal fracture are diagnostic. Strong pleochroism is visible to the naked eye in transparent specimens, shifting between dark brown and lighter yellowish brown as the crystal is rotated. To distinguish from smoky quartz, check the crystal cross-section: dravite is triangular while smoky quartz is hexagonal. Dravite also has higher specific gravity (2.98 to 3.20) compared to smoky quartz (2.65). Compared to schorl (black tourmaline), dravite is lighter in color when held to strong light and typically forms in metamorphic rather than pegmatitic settings. A streak test confirms both are white.