24 PRECIOUS STONES
instance
: very often, in a certain cavity, fluorspar has existed originally,
but, through some chemical means, has been slowly changed to quartz, so
that, as crystals cannot be changed in shape, we find quartz
existing—undeniably quartz—yet possessing the crystals of fluorspar ;
therefore the quartz becomes a pseudomorph, the condition being an
example of what is termed pseudomorphism. The actual cause of this
curious chemical change or substitution is not known with certainty,
but it is interesting to note the conditions in which such changes do
occur.
It
is found that in some cases, the matrix of a certain shaped crystal
may, after the crystal is dissolved or taken away, become filled by
some other and foreign substance, perhaps in liquid form; or a
crystalline substance may become coated or " invested" by another
foreign substance, which thus takes its shape ; or actual chemical
change takes place by means of an incoming substance which slowly
alters the original substance, so that eventually each is false and
both become pseudomorphs. This curious change often takes place with
precious stones, as well as with other minerals, and to such an extent
that it sometimes becomes difficult to say what the stone ought really
to be called.
Pseudomorphs
are, however, comparatively easy of isolation and detection, being more
or less rounded in their crystalline form, instead of having sharp,
well-defined angles and edges; their surfaces also are not good. These
stones are of little value, except in the specially curious examples,
when they become rare more by reason of their curiosity than by their
utility as gems.
Some also show cleavage planes of two or more
