,
It remains to us now to say a few words concerning the order that we
shall follow in the particular description of precious stones.
In
spite of all the discussions that have arisen on this subject, and the
great number of classifications presented by different authors who have
occupied themselves with the question, there does not exist, and there
cannot exist, any general and natural classification of precious
stones. The reason is very simple: these substances being what we may
call particular cases in nature, it is not possible to arrange them in series. By
choosing any one of their general characteristics, crystalline form,
refraction single or double, composition, or commercial value, &c,
the geometrician, the physicist, the chemist, and the merchant can
easily establish a classification answering more or less completely to
their special end; but this is not a natural classification.
Without discussing or criticizing the different methods proposed, we shall adopt in this book a classification based upon chemical composition. . If
there should be placed upon a table a specimen of every kind of
precious stone known at the present day, it would be possible to
separate them immediately, according to their chemical composition,
into three perfectly defined groups.
The first comprises a single precious stone, the diamond. Its constituent principle is carbon.
