are
preferred on account of their smaller liability to damage, cleavage
being less prominent than in the crystallised variety, or absent. Some
specimens of Bort and Carbonado also show a hardness superior to the
gem variety. In use, circular rings of steel, called "crowns," are
drilled on the flat surface with small holes, in each of which a small
piece of Bort is set, usually by drawing the mild steel partly over the
stone; so armed, the crown makes a rapid rock-cutting tool for boring
artesian wells or making prospecting bores. An advantage of this
.method is that solid cores are brought up and can be retained for
reference as to the rocks passed through. The use of Diamond as an
abrading agent has already been referred to under the cutting of gems;
it may be noted that the dust produced in the bruting of Diamonds is
more efficient on account probably of the flakes being taken off with a
definite relation to the directions of greatest hardness. Diamond
powder is used now not only in the grinding and polishing of Diamond,
but also, on account of the resulting increased speed of work, in the
grinding and slitting of such gems as Ruby, Sapphire, Chrysoberyl,
Topaz, etc.
Diamond
has been used for making small drills to pierce the other gem stones
for ornamental purposes, and for wire-drawing dies, and for drilling
the jewelled bearings for watches; also for use as a small milling
cutter with which to engrave gems and cut seals and cameos.
Dr.
Goring suggested its use for microscope lenses, and these lenses were
actually made by Pritchard ; but a difficulty arose from the specimens
used showing optical anomalies due to internal strain, also from the
great labour and cost of manufacture.
