In
crystalline form it is orthorhombic, and the habit is usually
prismatic, though in the variety Chiastolite it is more often acicular.
In composition it is a silicate of aluminium, A1203 SiO2
The variety Chiastolite shows a cross when cut in a plane normal to the
vertical axis, and when in large crystals is sometimes cut as a charm.
Like all strongly pleochroic minerals, the colour effect must be kept
in mind in cutting. A good specimen when cut resembles Alexandrite, but
is softer and less dense.
400. Cyanite.
Cyanite,
known also as Sappare and Kyanite, is closely related to Andalusite,
both in origin and composition; and, like Andalusite, though fairly
widely distributed, is but rarely found in a form suitable for gem use.
It is found in various shades of blue, green, and grey. The crystals
often show a good colour in the centre, but are pale or white at the
margins. The lustre is generally vitreous, but is pearly on the
cleavage surfaces ; transparent specimens are rare. The double
refraction is rather strong, the mean index being given as 1.72.
Pleochroism is well marked, and in the dichroscope images of two
different depths of blue are seen. It is infusible ; the specific
gravity is 3'58 to 3'68. The hardness shows an unusual
variation in the different directions, the least being between 4 and 5,
and the greatest as much as 7-1/4. It is a rather tough mineral.
There are two cleavages, one perfect, the other less so. In crystalline
form it is triclinic, and it usually occurs in embedded and elongated
forms, with no definite termination; the crystals are often bent. Most
of the gem varieties are supposed to have come from India, but the
exact locality is
