PRECIOUS STONES. 131
keenness
of sight, any doubtful mineral being adroitly tapped with the metal,
apparently with a view to determining its nature by its " feel." Now,
however, the gravel is passed over a vibrating table with a stepped
outline, the steps being covered with a thick layer of grease, to which
the Diamonds adhere (Claremont).
Applications.
Besides
its use in ornament there are certain technical purposes for which
Diamond is used. Perhaps the most familiar use is in cutting glass, and
it is a common belief that any bit of Diamond will do for this purpose;
but a glazier would soon tell one this is not the case. It is found
that the best form for this purpose is a natural crystal having
markedly curved faces, meeting in a not too obtuse edge. When such a
crystal is mounted in its holder and lightly applied to the glass with
the faces bounding this edge making equal angles with the glass, and
the glass itself making a tangent to the curved edge, a clear scratch
will result almost like a cut, though only about 1/200, inch in
depth. Even with a suitable Diamond it requires considerable skill to
cut glass properly, and different cutters have to be used for different
kinds of glass. In mounting the crystal, attention should be given to
the varying hardness in different directions, so that if a suitable
edge presents it may be utilised. "Wollaston ground other gem stones
into a form similar to that between two curved faces of Diamond and
found them satisfactory, though not so durable as the usual cutter.
Diamond is also largely used in the manufacture of rock drills. For this purpose the varieties Bort and Carbonado
k 2
