Stibnite. Image source: Mineralogical Information Institute.
Stibnite is a soft, gray mineral, antimony (II) sulfide (Sb2S3), the chief ore of antimony. It is found in low-temperature veins and rock impregnations, and occurs as orthorhombic system aggregates of either prismatic crystals or granular masses. It is opaque, sometimes irridescent, and gray or black in color. Much of the world's supply comes from China. Stibnite is used in the manufacture of fireworks and matches. Hardness 2, relative density 4.6.