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.
Color is normally green, but also can be brown, yellow, blue and/or purple.
Luster is vitreous or greasy to resinous.
Transparency crystals are transparent to translucent.
Crystal System tetragonal; 4/m 2/m 2/m
Crystal Habits include prismatic crystals with an overall square cross-section.
There is usually two sets of four sided prisms with one set being dominant.
The termination is usually a four sided pyramid that can be either steeply or gently sloped.
The faces of the pyramids align with the faces of the prisms making an edge that is perpendicular to the length of the crystal (as opposed to the tetragonal mineral apophyllite).
A pinacoid can truncate the pyramid or form the entire termination.
Massive forms are common and difficult to distinguish from massive garnet.
Cleavage poor, in one direction lengthwise.
Fracture is conchoidal to uneven.
Hardness is 6.5
Specific Gravity is 3.3 - 3.5
Streak is white.
Other Characteristics: striated lengthwise and is slightly pleochroic.