be, it is less valuable to the jeweler if it is not sufficiently hard to
withstand the abrasion to which articles of personal use are sub-
jected. (This statement does not apply to pearls and a few other
gems.) If not sufficiently hard, the stone may be badly scratched;
it will at least soon become dull by wear. An arbitrary scale of
hardness has been adopted, with the diamond, the hardest known
gem, placed at the top with a value of 10. The ruby and sapphire
come next, with a value of 9; the topaz is placed at 8, and quartz at 7.
(2) Specific gravity: This property in itself does not directly
affect the value of a gem stone, but it is very valuable in distinguish-
ing one stone from another.
(3) Crystalline form and cleavage:: The majority of gem
stones are crystalline in character. They lose their original crystal-
line form in cutting, but due to the crystalline structure, most gem
stones have definite cleavagee planes. This property is usually taken
advantage of in dressing stones preparatory to cutting.
(4) Color: The beauty and consequent value of gems depend
mainly on their color. Some stones, of course, as the diamond, re
valued for absence of color, but this is the exception. Color is
usually due to the presence of some foreign pigmentary matter,
usually in small proportions. The coloring matter may be evenly
distributed throughout the stone, or in regular zones, or in quite
irregular patches.
(5) Optical properties: Refraction. The brilliancy of a cut
stone depends on the amount of light reflected from its faces; in
the form known -as the "brilliant", the gem is so cut that much of
the incident light, after entering the stone and suffering refraction,
is totally reflected from the facets at the back. The amount of
light which is thus returned to the eye of the. observer will be
greater as the angle of total reflection, or critical angle, is smaller,
but this angle will be small if the refractive power of the stone is
great; so that the brilliancy depends directly on the refractivity.
Dispersion: The play of colors exhibited by a cut stone, often
known as its "fire", is due to the decomposition of the white light
which enters the stone, and is returned, by internal reflection, after
resolution into its colored components.
VALUE OF GEMS
Wholesale Prices:
Turquoise is worth up to $100 a pound at the mine, in the rough,
but usually is less valuable. Matrix material brings from $5 to $25
per pound. Cut matrix turquoise costs from 10 cents to $10 a
