THE BOOK OF DIAMONDS
ible
in fine gem diamonds, except under the microscope; but they pass
through the stone parallel to the crystal faces; all along these planes
the diamond can be split. If cleavage is too well developed, the planes
may appear as flaws. Other types of flaws are often seen in rough
diamonds, such as "knots" inclusions of other minerals, tiny spots of
black carbon, or bubbles of gas or liquids.
The
cubic diamonds are sometimes found in Brazil but are very rare among
the South African stones; and the dodecahedron is perhaps more common
in Brazil than elsewhere.
Diamonds,
frequently found in association with various minerals are universally
recognized as chief among precious stones. The majority of minerals are
found most commonly in masses which can with difficulty be recognized
as aggregates of crystalline grains, and occur comparatively seldom as
distinct crystals; but the diamond is almost always found in single
crystals; which show no signs of previous attachment to other stones.
The stones were, until the discovery of the South African mines, almost
entirely derived from sands or gravels, but owing to the hardness of
the mineral it is rarely, if ever, waterworn, and the crystals are
often very perfect. Small diamonds have been found in larger crystals.
The fact, and the statement, that the bursting of crystals is due to
inclusions of compressed gas, led Mr. Gardner F. Williams to question
the igneous theory of the slow growth of large crystals by accretion
instead of sudden solidification in a fused mass. One large diamond of
228 carats found several years ago, was formed around a small
68
