OF STONES FOR VARIOUS TYPES OF JEWELS, AS DETERMINED BY
HARDNESS, BRITTLENESS, AND CLEAVABILITY

HARD STONES NOT NECESSARILY TOUGH. As was suggested in the lesson on
hardness there is prevalent in the public mind an erroneous belief that
hardness carries with it ability to resist blows as well as abrasion.
Now that _it does not follow that because a precious stone is very hard,
it will wear well_, should be made plain. Some rather hard minerals are
seldom or never used as gems, in spite of considerable beauty and
hardness, because of their great brittleness. Other stones, while fairly
hard and reasonably tough in certain directions, have nevertheless so
pronounced a cleavage that they do not wear well if cut, and are
sometimes very difficult to cut at all.

In view of these facts it will be well to consider briefly what stones,
among those most in use, are sufficiently tough as well as hard, to give
good service in jewels, such as rings, which are subject to rough wear.
We may also consider those stones, whose softness, or brittleness, or
ready cleavability, requires that they should be reserved for use only
in those jewels which, because of their nature, receive less rough
usage.

In order to deal with the principal gems systematically, let us consider
them in the order of their hardness, beginning with the hardest gem
material known, which is, of course, diamond.

DURABILITY OF THE DIAMOND. Fortunately this king of gems possesses in
addition to its great hardness, considerable toughness, and although it
is readily cleavable in certain directions it nevertheless requires a
notable amount of force applied in a particular direction to cause it to
cleave. Although sharp knocks will occasionally flake off thin layers
from diamonds when roughly worn in rings, or even in extreme cases
fracture them, yet this happens but seldom and, as the enormous use of
the diamond in ring mountings proves, it is entirely suitable for that
purpose. It follows that, if a stone can stand ring usage, it can safely
be used for any purpose for which precious stones are mounted.

THE CORUNDUM GEMS. Next after the diamond in hardness come the corundum
gems, _i. e._, ruby, sapphire, and the series of corundum gems of colors
other than red and blue. These stones have no noticeable cleavage and
are exceedingly tough, for minerals, as well as very hard. We have only
to consider the use of impure corundum (emery) as a commercial abrasive
in eme