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 the quality of refraction is
due one of the chief charms of certain precious stones. It is not
necessary to explain here what refraction is, for everyone will be
familiar with the refractive property of a light-beam when passing
through a medium denser than atmospheric air. It will be quite
sufficient to say that all the rays are not equal in refractive power
in all substances, so
