azurite

A natural inorganic blue pigment with a slight greenish hue derived from the mineral of the same name. With its chemical composition of basic copper carbonate, 2CuCO3•Cu(OH)2, it is very similar to malachite. In damp conditions, it becomes green and turns into malachite, atacamite, or paratacamite. It can turn black when exposed to high temperatures or sulphur in the atmosphere and reacts to acids.