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The mineral pyrite, or iron pyrite, is iron disulfide, FeS<sub>2</sub>. It has isometric crystals that usually appear as cubes. The cube faces may be striated (parallel lines on crystal surface or cleavage face) as a result of alternation of the cube and pyritohedron faces. Pyrite also frequently occurs as octahedral crystals and as pyritohedra (a dodecahedron with pentagonal faces). It has a slightly uneven and conchoidal fracture, a hardness of 6–6.5, and a specific gravity of 4.95–5.10. It is brittle, meaning it breaks or powders easily. A major identifier, useful in the field, is the streak, as the powdered mineral smells of sulfur. Its metallic luster and pale-to-normal, brass-yellow hue have earned it the nickname fool's gold due to many miners mistakening it for the real thing. Ironically, small quantities of actual gold are sometimes found in pyrite. In fact, such auriferous pyrite is a valuable ore of gold.