mound,
and the diamond is placed at its summit. When the solder cools it
becomes rigid and the stone is held rigidly in position. Only a small
fraction of the stone is exposed at a time. Consequently while grinding
and polishing the numerous facets, frequent remountings have to be done.
At
present mechanical dops are widely used in lapping, and also in the
brillianteering of large stones. In the mechanical dop the stone is
rapidly held by prong and clamps, and the stone does not change its
position even when it gets heated during prolonged grinding. Remounting
is also easier in mechanical dops. The lapper also gets a full view of
the stone.
Semi-automatic
machines have now been devised for polishing diamonds—the Stern-Coleman
machine. It has micrometer gauges, degree finder and a tipping device.
The machine is set for a certain amount of grinding after which the
tripping device automatically removes the stone from the lap. 55-58% of
the stone used to be lost in cutting formerly. A 10% reduction or
saving has been made in the loss due to cutting by the introduction of
sawing. An attempt to save a large part of the stone may result in a
badly proportioned cut, which may very much reduce the value of the gem.
Cutting of otheb gems
The
methods employed differ from those of diamond due to different physical
properties and less hardness. The most important place has been
Idar-Oberstein in Germany, where most of the people are employed in
this industry. Those who cut gems other than diamonds are called lapidaries, but
again a difference exists between gem-cutters and lapidaries. The
gem-cutters use sandstone wheels, which are run by electric or water
power and cut agate, quartz, opal, etc., which have a hardness of 7 or
less. Lapidaries use carborundum wheels and electric power and
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