and that a beam of light reflected at a certain angle will be
again reflected if the two plates are parallel to each other, but
not if they are perpendicular to each other ; this beam is said
to be polarized. In some double-refracting crystals, the two
opposite polarized beams of light are of different colors.
Classification. — Hardly any two writers concur in the same
system of classification, and perhaps in no other department
of scientific knowledge have there been so many arbitrary
arrangements of a subject as in that of precious stones. This
may have arisen from a difference of opinion in regard to their
true character, and ignorance of their chemical constituents
and the laws which govern their crystalline forms. It has been
said that no strictly scientific classification of gems is possible ;
but the nearest approach to it can be reached through their
chemical properties, and habits of crystallization. Grouping
them according to color, important for some purposes, is the
most striking method, and the one most frequently employed
by the ancients, but it is entirely misleading in regard to
their real nature. The practical artist classes them in reference to color, transparency, brilliancy, and some other attributes ; the dealer ranks them in the order of their commercial
value and the varying moods of fashion. Another arrangement
is to call all those of superior excellence "oriental," though
they may never have been brought from the east, and those of
inferior quality "occidental," without regard to the place of
their origin.
As it is impossible to classify precious stones in any regular
system depending upon their beauty, color, transparency, or
any other external quality, since the same species often presents
a great diversity in these attributes, that method of grouping
them according to their chemical composition is probably the
best which has yet been employed. In examining a gem-min-
