Although
nature always follows certain laws, her productions from the materials
at hand are so many and varied, her similitudes so interwoven, that
the classifications, and the standards created by which to judge them,
cannot be absolute. For reasons and from causes too involved and
intricate for the mind of man to disentangle and detail them, all the
features of the different stones are at one time or another changed or
modified. The proportions in the chemical compositions vary; the
crystallization presents different forms; the specific gravity varies
slightly; one coloring matter has been used now, another then; at one
time, conditions being favorable, more perfect work has been done than
at another. For these reasons, all tables of hardness, specific
gravity, composition, etc., are theoretical, and while they represent
conditions approximately, and serve to define the relations and
differences existing between one mineral and others, no stone is always
exactly as tabulated, and must be recognized by a general conformity to
the descriptions given, which are composite, not individual
photographs of the things of which they treat.
Under
the superficial qualities patent to every eye these individual and
combined characteristics which differentiate the various precious
stones have provoked the scientist to many discoveries, and a knowledge
of them serves to increase the wonder and admiration of men for these
beautiful productions of nature.
They are found in crystallized or amorphous conditions. The plans of crystallization are,—
1. Those having the axes equal, as the isometric, or cubic. This includes the diamond, garnet, and spinel.
2.
Those having only the lateral axes equal, as the tetragonal or
pyramidal, and hexagonal or rhombohedral, and comprise the amethyst,
aquamarine, emerald, rock crystal, ruby, sapphire, topaz, tourmaline,
and zircon.
3. Those having the axes unequal, as the orthorhombic
