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