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CHAPTER VII.

DESCRIPTION OF PRECIOUS STONES.


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