Formation of Diamonds - Common Forms
The most common forms of diamonds are the regular octahedron and the rhombic dodecahedron ; the former bounded by eight equilateral triangles, and the latter by twelve rhomb's, or lozenge-shaped surfaces. 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 infrequently 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 ".
Formation of Diamonds - The Surface
The surface of a crystal of Diamond is generally smooth ; but it is sometimes indented with triangular impressions, and in certain cases has lines parallel to the edges of the octahedral faces. Some Diamonds present a rough surface, resembling poorly polished glass, and are often dull, as though covered with a thin coating of gum. These generally cut into very fine white stones.
Formation of Diamonds - Bort
The Diamond is occasionally found in concretionary crystalline forms, which pass under the name of Bort; while another variety termed Carbonado of brownish-black colour, is so indistinctly crystalline as to be often regarded as compact.
Formation of Diamonds - Cleavage
The Diamond presents a perfect cleavage, parallel to the faces of the octahedron, which is its primary form. The fracture of the Diamond, apart from its cleavage, is conchoidal, and here and there the stone is liable to split off in fragments.
