Fornacite is a rare lead, copper chromate arsenate hydroxide mineral with the formula: Pb₂Cu(CrO₄)(AsO₄)(OH). It forms a series with the phosphate mineral vauquelinite. It forms variably green to yellow, translucent to transparent crystals in the monoclinic - prismatic crystal system. It has a Mohs hardness of 2.3 and a specific gravity of 6.27.