of precious stones as are perceptible to the eye unaided by optical
apparatus, but trained to keenness of vision, afford valuable means of
discriminating precious stones from one another, but do not exhaust
such means. Indeed, such mechanical properties as hardness and specific
gravity are of the greatest use in determining the species of a stone,
and are more commonly available than the majority of optical tests.
Optical properties must, however, ever hold a chief place in all
artistic classifications of precious stones, so that it will not be
unadvisable to begin the present chapter by a synopsis of the most
obvious characters of this class. They may be arranged in the form of a
tabular view, the use of which is twofold, enabling us to define the
several optical properties found in gem-stones, and also to appreciate
their artistic capabilities. We arrange these optical (or mainly
optical) qualities under the general heads of " Surface " and "
Substance : "