Galena, a lead sulfide, PbS, is the most important ore mineral of lead. Its metallic, lead gray cubic crystals (isometric system) and cubic, perfectly cleavable masses are distinctive and characteristic. Streak lead gray. 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 an ore.