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SECTION II.
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CHAPTER I.
THE DIAMOND.
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IAMONDS,
as they occur in Nature, usually but not invariably present the form of
crystals, more or less regular and perfect in their development. These
forms belong to the group of geometrical solids known to
crystallographers as the Cubic or Tesserai or Isometric system.
The most common forms are the regular octahedron and the rhombic
dodecahedron ; the former bounded by eight equilateral triangles, and
the latter by twelve rhombs, or lozenge-shaped surfaces. It is notable
that the faces of the crystals are often more or less curved, or
convex, whilst those of other crystalline bodies, with few exceptions,
are flat. Not unfrequently the Diamond takes the form of a six-faced
octahedron, which, by the rounding of its eight-and-forty faces becomes
almost spherical or approaches a small ball in shape. In some cases the
crystals are curiously " twinned " or " macled."
Groups
of crystals, dodecahedra as well as octahedra, are not rare ; there is
for instance, a very fine specimen of such a mass of coalesced
octahedra in the Royal Mineral Museum at Dresden. In the Vienna
Collection there is a Diamond which has, enclosed within itself,
another similarly-crystallised Diamond of a yellow-colour ; and
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