o individuals
interpenetrating and projecting through each other. If this be so the
form of the diamond is really the tetrahedron (and the various figures
derived symmetrically from it) and not the octahedron. Fig. 5 shows how
the octahedron with furrowed edge may be constructed from two
interpenetrating tetrahedra (shown in dotted lines). If the grooves be
left out of account, the large faces which have replaced each
tetrahedron corner then make up a figure which has the aspect of a
simple octahedron. Such regular interpenetrations are known in
crystallography as "twins." There are also twins of diamond in which two
octahedra (fig. 6) are united by contact along a surface parallel to an
octahedron face without interpenetration. On account of their
resemblance to the twins of the mineral spinel (which crystallizes in
octahedra) these are known as "spinel twins." They are generally
flattened along the plane of union. The crystals often display
triangular markings, either elevations or pits, upon the octahedron
faces; the latter are particularly well defined and have the form of
equilateral triangles (fig. 7). They are similar to the "etched figures"
produced by moistening an octahedron of alum, and have probably been
produced, like them, by the action of some solvent. Similar, but
somewhat different markings are produced by the combustion of diamond in
oxygen, unaccompanied by any rounding of the edges.

[Illustration: FIG. 1.]

[Illustration: FIG. 2.]

[Illustration: FIG. 3.]

[Illustration: FIG. 4.]

[Illustration: FIG. 5.]

[Illustration: FIG. 6.]

[Illustration: FIG. 7.]

Diamond possesses a brilliant "adamantine" lustre, but this tends to be
greasy on the surface of the natural stones and gives the rounded
crystals somewhat the appearance of drops of gum. Absolutely colourless
stones are not so common as cloudy and faintly coloured specimens; the
usual tints are grey, brown, yellow or white; and as rarities, red,
green, blue and black stones have been found. The colour can sometimes
be removed or changed at a high temperature, but generally returns on
cooling. It is therefore more probably due to metallic oxides than to
hydrocarbons. Sir William Crookes has, however, changed a pale yellow
diamond to a bluish-green colour by keeping it embedded in radium
bromide for eleven weeks. The black coloration upon the surface produced
by this process, as also by the electric bombardment in a vacuum tube,
appears t