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been
collected and analyzed, and found to be carbon dioxide, just as would
result from the combustion of other forms of carbon. If protected from
the air or free oxygen, the diamond can be exposed to high heat without
change.
Being
a crystallized substance and excessively hard, the diamond is usually
found in the form of more or less perfect crystals. These have forms
such as the cube, octahedron, etc., which belong to the isometric
system, and it is in this system that the diamond crystallizes. The
crystals do not possess, however, the highest isometric symmetry, but
belong to the class designated by Groth as hexakistetrahedral,
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rounded
forms, and twin crystals are common. Although so excessively hard, the
edges of the crystals, as found in the beds of streams, are often
rounded from the wear of the other pebbles, probably chiefly quartz.
Only the wear of centuries could produce such a result, however; for,
as is well known, it is only with its own dust that the diamond can be
abraded to any appreciable degree by any of the means now used for
cutting it.
One
important property of crystallized diamond is that of cleavage parallel
to the faces of the octahedron. This cleavage is of much service in
preparing the gem for cutting, as by taking advantage of it, broad,
flat surfaces can be obtained without grinding. This property also
distinguishes diamond from quartz, for which its crystals, as found in
sands, are sometimes mistaken. Quartz has no cleavage. The fracture of
the two minerals is the same however, being conchoidal.
The massive forms of the diamond known as bort and carbonado
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