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 systems, and others show
a crystalline structure comprised of several systems. Thus calcspar is
in the 2nd, or hexagonal, whilst aragonite is in the 4th, the rhombic,
system, yet both are the same substance, viz.:--carbonate of lime. Such
a condition is called dimorphism; those minerals which crystallise in
three systems are said to be trimorphous. Those in a number of systems
are polymorphous, and of these sulphur may be taken as an example, since
it possesses thirty or more modifications of its crystalline structure,
though some authorities eliminate nearly all these, and, since it is
most frequently in either the 4th (rhombic) or the 5th (monoclinic)
systems, consider it as an example of dimorphism, rather than
polymorphism.

These varieties of cleavage affect the character, beauty and usefulness
of the stone to a remarkable extent, and at the same time form a means
of ready and certain identification and classification.

CHAPTER V.

PHYSICAL PROPERTIES.

C--LIGHT.

Probably the most important of the many important physical properties
possessed by precious stones are those of light and its effects, for to
these all known gems owe their beauty, if not actual fascination.

When light strikes a cut or polished stone, one or more of the following
effects are observed:--it may be transmitted through the stone,
diaphaneity, as it is called; it may produce single or double
refraction, or polarisation; if reflected, it may produce lustre or
colour; or it may produce phosphorescence; so that light may be (1)
transmitted; (2) reflected; or produce (3) phosphorescence.

(1) TRANSMISSION.--In transmitted light we have, as stated above, single
or double refraction, polarisation, and diaphaneity.

To