The Sulfide Mineral, Galena, a lead sulfide, PbS, is the most important ore mineral of lead. It can contain 86% lead. Its metallic, lead-gray cubic crystals (isometric system) and cubic, perfectly cleavable masses are distinctive and characteristic. Hardness is 2.5, streak lead gray, and specific gravity 7.4-7.6. Galena is a widespread mineral deposited by hydrothermal solutions as large, irregular masses in dolomitized limestones and in zones of contact metamorphism and as veins in volcanic rocks. It often contains enough silver to be mined as a silver ore.