ORIGIN, PROPERTIES, CLASSIFICATION, ETC. II
and gems in a strictly scientific sense. The name "precious "
applied to a mineral refers to only a few species, generally distinguished by superior transparency, lustre, color, hardness,
and some other characteristics ; while " gem " is a term which
embraces a wider range, and comprises a larger variety of
materials used for personal decoration. In a popular sense,
however, precious stone and gem are nearly identical, and
include several substances not mineral, and others, which are
wanting in some of the qualities considered essential in an
•ornamental stone of the first class.
Writers on precious stones differ materially in the classification and arrangement of their properties, some of the older
mineralogists making color the test of their distributive order,
while modern scientists class them according to their chemical
constituents, which consist largely of carbon, aluminum, silicon,
magnesium, glucinum, zirconium, and iron, with alkalies for
solvents. The excellence of precious stones, it has been said,
depends not so much upon their composition as upon the complete solution and combination of their constituents.
Their physical properties are color, lustre, hardness, specific
gravity, refraction, polarization, fusibility, combustibility, phosphorescence, and crystallization.
Color. — This is one of the most striking and important
qualities of ornamental stones, and constitutes their most
attractive feature, always excepting colorless diamonds, and
some other species of the first rank. It affords, also, some of
the most interesting phenomena connected with these marvels
of creation. They may exhibit only one color, and are, therefore, monochroic ; they may have more than one, when they
are called pleochroic ; they may be opalescent, or prismatic,
and display all the colors of the rainbow ; again, they may
Teflect rays differing from the color of the crystal, when they
