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In
alluvial stones, the original shape is not present due to wear and
tear. The mine diamonds are often beautiful with triangular depressions
or 'trigons' whose angles point to the octahedral edges. This enables
the determination of orientation in broken crystals. The trigons are
'etched figures'. In dodecahedrons, striations parallel to the longer
diagonals of the rhombs are seen.
Twinning.—Octahedral
and dodecahedral twins are common. The spinel twin is also called the
'macle'. macles are rounded like simple crystals. Illusory twinning,
'trigons', are depressions. Elevations rising from a triangular base on
the octahedral faces are also oriented conformably with the face. These
elevations tend to arrange themselves symmetrically about the coigns of
the crystal., They are nearly colourless and lose a great deal of
weight in cutting.
Specific gravity.—The specific gravity of diamond is 3.5-3.6 Pure diamond has 3.53, the black variety 3.45
3.52 and bort 3.6. Hardness is 10 on the Moh's scale of
hardness.
It is the hardest mineral known, but it does not mean that it is twice
as hard as apatite. In a scale representing relative hardness, the
value for diamond would be 100 rather than 10. Diamonds from various
areas are found to have varying hardness. The diamonds from Borneo and
Australia are said to be harder than those from Kimberley. Perhaps some
impurity might contribute to increased hardness. Hardness must not be
confused with toughness, as diamonds are brittle and easily chipped. It
has.also been found that hardness in diamonds varies with the different
crystal faces and also with the direction of the faces. This fact is
known to diamond cutters, as also the following: (a) faces
parallel to the cube are easy to polish, as they are parallel to two
crystal axes; (fe) rhombic dodecahedral faces are parallel to one
crystal axis, and hence less easy to polish them than the faces
parallel to the cube; and (c) the octahedral faces being
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