Idocrase, also known as versuvianite (because it was first found on Mount Vesuvius), is an orthosilicate mineral, consisting of hydrous calcium, iron, magnesium, aluminum silicate. Found in impure limestone, it forms tetragonal system crystals and is glassy green, brown, yellow, or blue. Its transparent brown or green crystals are cut as gems. Hardness 6.5; relative density 3.4.