Vesuvianite ( Idocrase )
Structure : TetragonalHardness : 6.5
Specific Gravity : 3.4
Refractive index : 1.70 - 1.7
Doubly Refracting (bi-refraction) : 0.005
Vesuvianite, also known as Idocrase, is a fascinating mineral found originally on the volcano, Mt Vesuvius, hence one of the names. The other name, idocrase, is from the greek and means mixed form, an allusion to its crystals showing a mixture of other mineral forms. The crystals belong to the tetragonal symmetry class and show a square cross-section perpendicular to the long axis. Tetragonal crystals are uncommon and vesuvianite fortunately produces some very nicely shaped crystals.
Oddly enough, some of its structure is similar to that of grossularite, a garnet, which is an isometric mineral. Its structure is also interesting in that it is composed of both SiO4 groups and Si2O7 groups. Vesuvianite could be classified as a nesosilicates because of the SiO4 groups, but the higher organization of the Si2O7 groups puts it in the sorosilicate subclass.
Vesuvianite forms as a result of contact metamorphism on impure limestones and is usually found with other exotic minerals. A massive green gem variety is called californite from where it is found. It is a somewhat rare and beautiful mineral that can rival many other minerals for interest among mineral collectors.