examination
of many thousands of crystals of Barytes, has invariably found one or
two crystal forms, when present, to be curved, and this in a mineral
that may show over thirty forms on one crystal, all the others being
remarkably flat. This would rather suggest that the curvature, which is
undoubtedly developed at the time of the growth of the crystal, is due
not so much to the surrounding conditions as to some special surface
force acting on particular planes of the crystal in a much greater
degree than on others. This curving of the faces may in some types of
crystal lead to an almost spherical form, as in No. 5 of the drawings
from the Eaith collection (Figs. 10 and 11) ; again, it leads to great
difficulty in the exact measurement of the angles on Diamond crystals,
and for this reason the interesting question of the exact group of
symmetry to which the mineral belongs is still unsettled. Another
characteristic of the crystals is the presence of numerous striatums on
some of the faces, usually parallel to the intersection with the
octahedron. Two crystal forms show characteristic markings on their
faces : thus the cube faces have little depressions, as if a truncated
four-sided pyramid had been pressed into them smaller end first; the
faces of the octahedron, too, show sunk triangular pits with the
triangular outline inverted in relation to the outline of the
octahedral face.
Although
Diamond crystals are as a rule idiomorphic, this is not invariably the
case, some being found impressed by other crystals, such as Quartz ;
inclusions of a mineral which is very likely Chlorite, and more rarely
of Haematite, occur; some crystals contain innumerable minute cavities
; Diamond also occurs included in Quartz and Anatase.
Crystals of cubic habit are most frequently found in
