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182 THE DIAMOND MINES OF SOUTH AFRICA
from
its centre to its perimeter. By means of belts beneath the table, the
grinding wheel is made to turn at a rate from three to four thousand
revolutions a minute. Diamond dust mixed with olive oil is applied to
the upper face of the wheel, and against this erosive surface is held
the diamond to be ground or cut.
For
this object the diamond is set in a fusible solder on the end of a
copper cupel which is held firmly against the surface of the wheel by a
small projecting arm and clamp. By adjusting this holder, the lapidary
presses the exposed face of the stone on the revolving wheel until the
desired amount of material has been ground away and the proper angles
turned. Such work in its finishing stages cannot be intrusted to a tyro
or experimenter. Unusual patience and steadiness of nerve are required
for such a task. When the facet is finished, the workman wipes the dust
off and tests its smoothness and finish, after which he resets the
diamond, leaving the uncut facet exposed which he intends to cut next.
Most
of the " skill " facets and " underskill " facets are made by grinding,
while the lozenge and larger faces are first shaped, when possible, by
cleaving. If the stone is thick enough to form a brilliant, the
lapidary first forms the table, and then successively the adjacent
facets and lozenges. The table must be absolutely flawless and smooth,
while all the surrounding facets in an ideal brilliant must hold the
same precise angles and have their shape correspond to the thousandth
of an inch. After completing the bezel, the pavilion is next developed.
The underskill facets of the pavilion must match exactly at the girdle
with those of the bezel, and the girdle when finished should be as
sharp as a knife. Some lapidaries leave the girdle blunt, but with a
great sacrifice of brilliancy in the gem. The triangles and lozenges of
the pavilion must, of course, be much larger than those of the bezel.
There
is a still simpler method of cutting diamonds by a device attributed to
Berquem. Two uncut stones are cemented into the ends of two sticks
resembling penholders in shape. Then the operator grasps these handles
and presses the stones
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