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CHAPTER VII.
DESCRIPTION OF PRECIOUS STONES.
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DIAMOND.
There are
three characters which unite to place the diamond in a unique position
amongst precious stones. It is the only gem which is combustible ; it
is the hardest of all minerals ; it exerts upon light the most
energetic refractive and dispersive power.
The
diamond belongs to the cubic or isometric system, and usually occurs in
the form of an octahedron, or in combinations in which the cube, the
dodecahedron, and the tetrahedron are involved. The faces of these
forms are commonly curved : macled and hemitropic associations of
crystals are of frequent occurrence.
The
diamond is easily cleaved in directions parallel to its octahedral
faces. Its fracture is conchoidal. Its hardness is 10.
The
lustre both of natural and artificial surfaces of diamond is peculiarly
brilliant, approaching that of such a metal as silver. This
characteristic lustre, which is shared to some extent by sphene,
jargoon and garnet, is known as adamantine—it lies between the
metallic and the resinous lustres. The peculiar brilliancy of diamonds
results in part from the total reflection of light from their internal
faces when the incident light strikes them at an angle
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