Nissonite is a very rare copper phosphate mineral with the formula: Cu<sub>2</sub>Mg<sub>2</sub>(PO<sub>4</sub>)<sub>2</sub>(OH)<sub>2</sub>÷5H<sub>2</sub>O. 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.
Nissonite was discovered in 1966 and was named after William H. Nisson (1912âÂÂ1965). It is from Llanada copper mine, near Llanada, San Benito Co., California.