PRECIOUS STONES. 63
the required angle. In this way much more precise work is possible.
"When
all the facets have been polished, the stone is cleaned by treatment
with a softer material, such as bone ash or tripolite.
The
treatment of softer stones is somewhat different. Some of them, as
Topaz and, to a lesser degree, Beryl may be partly formed by cleavage,
but, in most cases the form is given entirely by grinding. For the
varieties of Corundum, for Chrysoberyl, Emerald and other hard stones,
the lap is made of iron, copper, tin, pewter, or lead ; usually the
harder the gem, the harder the disc used. The abrasive material is now
often Diamond, for though this means a greater cost in material, there
may be a greater saving in total cost of the work from the greater
rapidity of grinding: the coarser varieties of Corundum known as Emery
are also used, and now the artificially prepared compound of carbon and
silicon, called carborundum, is extensively employed. Carborundum is
made by the treatment of sand and coke in the arc of an electric
furnace, and can be produced cheaply in considerable quantity; in
commerce it is found in brilliant hexagonal crystals of a rich yellow
colour, often merging into the blue known as " electric blue," giving
the crystals very much the appearance of the Ilavmatite crystals from
Elba. Carborundum has the further advantage of being very hard—much
harder than the Sapphire—and yet easily reducible to powder on account
of its brittleness. The softer abrasives are mixed with water instead
of with oil; sometimes powdered Garnet and Topaz are used.
The gem is cemented to the end of a holder of wood or
