Habit: Occurs as dark green, bluish green, blue to indigo, yellow to brown octahedral crystals, also irregular grains and massive. Translucent to nearly opaque, vitreous Luster.
Environment: Found in crystalline schists, in Li-rich granitic Pegmatites and in contact metamorphosed limestones. Also in high-temperature replacement ore deposits in schists or marbles or quartzose rocks.
Etymology: Named for Johann Gottlieb Gahn (1745-1818), Swedish chemist and mineralogist who discovered manganese.