equally
inclined to three crystal axes and not being parallel to any crystal
axis are the most difficult to polish. The change in hardness from one
face to another or from one direction to another is not abrupt but
gradual.
Cleavage
is parallel to the octahedral face (111) and the mineral breaks along
the four perfect cleavages parallel to the face of the octahedron.
Dodecahedral cleavage is imperfect. Fracture is irregular and may vary
also from conchoidal to splintery. Colour may vary from yellow, brown,
grey and black. Each of them may have several grades. The bort belongs
to the grey species and occurs in round and shapeless lumps and
fragments. It has crystalline structure. The following are the
varieties of diamond: (1) diamond proper, (2) bort or bortz, (3)
ballas, and (4) carbonado.
Optical properties.—The
crystals may be clear and transparent with a brilliant lustre on the
surface, but may be also translucent and opaque. The transparent stones
are valued as gems. The_lustre js adamantine, but an uncut diamond may
be dull or greasy. Carbonado may have a dull lustre. The full
properties diamond, i.e. brilliancy, fire and play of .colours, for
which the diamond is highly valued as a gem, are brought out by cutting
and polishing. Diamonds cut for jewellery are known as 'brilliants'.
These valuable qualities of diamond are due to its optical characters,
e.g. refractive index 2-402 for red and 2.465 for violet and their
difference 0-063 the dispersion. It is singly refractive or isotropic,
but anomalous double refraction may be noted due to internal strains.
Double refraction may also be noted surrounding enclosures or cracks.
In respect of play of colours pris-matic) the Indian stone rank highest, followed by Brazilian and the Capa diamonds
Inclusions
of graphite are most common. Other minerals found as inclusions are
ilmenite, chromite, magnetite and hematite. Garnet also occurs as
rounded grains or frequent
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