o be due to a conversion of the surface film into graphite.
Diamond may break with a conchoidal fracture, but the crystals always
cleave readily along planes parallel to the octahedron faces: of this
property the diamond cutters avail themselves when reducing the stone to
the most convenient form for cutting; a sawing process, has, however,
now been introduced, which is preferable to that of cleavage. It is the
hardest known substance (though tantalum, or an alloy of tantalum now
competes with it) and is chosen as 10 in the mineralogist's scale of
hardness; but the difference in hardness between diamond (10) and
corundum (9) is really greater than that between corundum (9) and talc
(1); there is a difference in the hardness of the different faces; the
Borneo stones are also said to be harder than those of Australia, and
the Australian harder than the African, but this is by no means certain.
The specific gravity ranges from 3.56 to 3.50, generally about 3.52. The
coefficient of expansion increases very rapidly above 750 deg., and
diminishes very rapidly at low temperatures; the maximum density is
attained about -42 deg. C.

The very high refractive power (index = 2.417 for sodium light) gives
the stone its extraordinary brilliancy; for light incident within a
diamond at a greater angle than 24-1/2 deg. is reflected back into the
stone instead of passing through it; the corresponding angle for glass
is 40-1/2 deg. The very high dispersion (index for red light = 2.402,
for blue light = 2.460) gives it the wonderful "fire" or display of
spectral colours. Certain absorption bands at the blue end of the
spectrum are supposed to be due to rare elements such as samarium.
Unlike other cubic crystals, diamond experiences a diminution of
refractive index with increase of temperature. It is very transparent
for Rontgen rays, whereas paste imitations are opaque. It is a good
conductor of heat, and therefore feels colder to the touch than glass
and imitation stones. The diamond has also a somewhat greasy feel. The
specific heat increases rapidly with rising temperature up to 60 deg.
C., and then more slowly. Crystals belonging to the cubic system should
not be birefringent unless strained; diamond often displays double
refraction particularly in the neighbourhood of inclusions, both liquid
and solid; this is probably due to strain, and the spontaneous explosion
of diamonds has often been observed. Diamond differs from graphite in
be