Nissonite is a very rare copper phosphate mineral with formula: Cu₂Mg₂(PO₄)₂(OH)₂·5H₂O. It crystallizes in the monoclinic crystal system typically as crusts, tabular crystals, and diamond-shaped crystals. The color is blue-green. It has a light green streak, a Mohs hardness of 2.5 and a specific gravity of 2.73. Cleavage is (100) distinct.