icularly is this seen in
the diamond "brilliant," which plainly evidences the four cleavage
planes. These cleavage planes and their number are a simple means of
identification of precious stones, though those possessing distinct and
ready cleavages are extremely liable to "start" or "split" on these
planes by extremes of heat and cold, accidental blows, sudden shocks and
the like.

In stones possessing certain crystalline structure, the cleavage planes
are the readiest, often the only, means of identification, especially
when the stones are chemically coloured to imitate a more valuable
stone. In such cases the cleavage of one stone is often of paramount
importance in testing the cleavage of another, as is seen in the
perfection of the cleavage planes of calcite, which is used in the
polariscope.

It sometimes happens, however, that false conditions arise, such as in
substances which are of no form or shape, and are in all respects and
directions without regular structure and show no crystallisation even in
the minutest particles; these are called amorphous. Such a condition
sometimes enters wholly or partially into the crystalline structure, and
the mineral loses its true form, possessing instead the form of
crystals, but without a crystalline structure. It is then called a
pseudomorph, which is a term applied to any mineral which, instead of
having the form it should possess, shows the form of something which has
altered its structure completely, and then disappeared. For 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