rs so that the effect may be seen
equally over all parts of the stone, as set.

Cat's-eye and tiger's-eye owe their peculiar appearance to the presence,
within them, of many fine, parallel, silky fibers. The quartz cat's-eye
was probably once an asbestos-like mineral, whose soft fibers were
replaced by quartz in solution, and the latter, while giving its
hardness to the new mineral, also took up the fibrous arrangement of the
original material. The true chrysoberyl cat's-eye also has a somewhat
similar fibrous or perhaps tubular structure. Such stones, when cut _en
cabochon_, show a thin sharp line of light running across the center of
the stone (when properly cut with the base parallel to the fibers).
This is due to reflection of light from the surfaces of the parallel
fibers. The line of light runs perpendicularly to the fibers.

In these cases (opals, starstones, moonstones, and cat's-eyes) the
individual stone is usually easily distinguished from other kinds of
stones by its peculiar behavior towards light. However, it must be
remembered that other species than corundum furnish starstones (amethyst
and other varieties of quartz, for example), so that it does not follow
that any starstone is a corundum gem. Also the more valuable chrysoberyl
cat's-eye may be confused with the cheaper quartz cat's-eye unless one
is well acquainted with the respective appearances of the two varieties.
Whenever there is any doubt other tests should be applied.

For further account of luster and other types of reflection effects see
_Gem-Stones_, by G. F. Herbert-Smith, Chapter V., pp. 37-39, or _A
Handbook of Precious Stones_, M. D. Rothschild, pp. 17, 18.

LESSON VIII

HARDNESS

Another property by means of which one may distinguish the various gems
from each other is _hardness_. By hardness is meant the ability to
resist scratching. The term "hardness" should not be taken to include
toughness, yet it is frequently so understood by the public. Most hard
stones are more or less brittle and would shatter if struck a sharp
blow. Other hard stones have a pronounced _cleavage_ and split easily in
certain directions. True hardness, then, implies merely the ability to
resist abrasion (_i. e._, scratching).

Now, not only is hardness very necessary in a precious stone in order
that it may _receive_ and _keep_ a fine polish, but the degree in which
it possesses hardness as compared with other materials of known
hardness may be m