PRECIOUS STONES. 41
be
seen in most rough precious stones ; it must be disÂtinguished from
cleavage (q.r.) ; fractured surfaces, though they approach geometrical
planes in some cases, are never true planes. "When the fracture is flat
or nearly flat it is said to be " even " ; this is seen in some
Jaspers. Should it be rougher and covered with minute points it is "
uneven," as in Lapis Lazuli; when still more rough it is "hackly." When
the broken surface shows the smooth curves so well seen in a broken
piece of thick glass, the fracture is " conchoidal" (i.e. shell-like),
this is well seen in Quartz and many of the gem stones. Where a gem
stone has been damaged, as by a blow, but without separation of the
fragments, a crack may often be seen reflecting beautiful prismatic
colours (c.f. refraction). This is occasionally made use of in
stones that have no intrinsic beauty of colour as in Quartz; but in a
highly refracting gem any flaw of the sort, especially if situated at
the back part of the cut stone, greatly detracts from the brilliance,
so that when a flaw develops before the stone is cut, the size of the
finished stone and the direction of the cuts are arranged- to eradicate
such blemishes. This was exemplified in the case of the Koh-i-noor,
which as exhibited at the Crystal Palace weighed 186-1/16 carats, but which was afterwards re-cut by Messrs. Garrard, when it weighed 102-1/4 carats.
Brittleness
depends very largely on the grosser structure of the stone ; where this
is minutely fibrous, as in Crocidolite or Malachite, there may be
considerable tenacity, though the hardness of these two minerals is
very different; at the same time the brittleness depends partly on the
hardness, and also on the presence or absence of a cleavage.
