The specific gravity is high for a gem stone, that of crystals being 3'516 to 3-525; the variety Carbonado is, however, rather less dense, 3-15—3-29, while Bort is almost the same as the crystallised variety, 3"5.
"When broken, if a fracture is developed, it is seen to be conchoidal, but cleavage is much more often seen than fracture.
The
cleavage of Diamond is parallel to the faces of the octahedron, and is
highly perfect, so that the cleavage planes appear bright, smooth and
regular; cleavage is obtained with great ease by the means described
under gem-cutting, and use is very frequently made of this property in
the cutting of the gem into brilliants, since this particular shape may
be said to be derived from a regular octahedron. Hence, whatever the
external form of a rough Diamond, if it is a single crystal or a
portion of one, it can be reduced easily to a suitable form for
cutting. In cases where no actual crystal faces are to be seen on the
rough stone a careful examination will usually reveal some of the
octahedral cleavage planes, and thus we can determine in what direction
the remaining planes of the octahedron may be produced. To recognise
these planes when unequally developed requires some practice, and it is
well worth while to obtain some lumps of Fluor Spar, which has the same
cleavage, and to practice the recognition of existing cleavages and the
production of fresh ones on this cheap material, which can easily be
obtained in single crystals the size of one's fist. Twinned crystals of
Diamond cannot be cleaved into an octahedron by simple cleavage.
In hardness Diamond exceeds all other known substances, and on Mohs' scale it is therefore numbered 10. The
