USGT - Graphite, Classified as a semi-metal, this mineral, also called black lead, is relatively common. It naturally occurs in metamorphic rocks formed by heat and pressure, igneous rocks formed from volcanic eruptions and meteorites. High-grade crystalline flake deposits are composed of flat, hexagonally edged particles. Amorphous deposits are fine particles that result from natural thermal processing. Lump graphite occurs as veins in rock fissures created through natural hydrothermal processes.