rite) are all
softer than quartz (7) but harder than feldspar (6); hence we say they
are 6-1/2 in hardness. Beryl (aquamarine and emerald), garnet
(almandine), and zircon are rated 7-1/2 in hardness, being softer than
true topaz but harder than quartz. A table of the hardness of most of
the commonly known gem-stones follows this lesson.

Having now an idea of what hardness means and how it is expressed, we
must next inquire how one may make use of it in identifying unknown
gems.

HOW TO APPLY THE HARDNESS TEST. In the first place, it is necessary to
caution the beginner against damaging a fine gem by attempting to test
its hardness in any but the most careful manner. The time-honored file
test is really a hardness test and serves nicely to distinguish genuine
gems, of hardness 7 or above, from glass imitations. A well-hardened
steel file is of not quite hardness 7, and glass of various types while
varying somewhat averages between 5 and 6. Hence, glass imitations are
easily attacked by a file. To make the file test use only a _very fine_
file and apply it with a light but firm pressure lengthwise along the
girdle (edge) of the unset stone. If damage results it will then be
almost unnoticeable. Learn to know the _feel_ of the file as it takes
hold of a substance softer than itself. Also learn the _sound_. If
applied to a hard stone a file will slip on it, as a skate slips on ice.
It will not take hold as upon a softer substance.

If the stone is set, press a sharp corner of a broken-ended file gently
against a _back_ facet, preferably high up toward the girdle, where any
damage will not be visible from the front, and move the file very
slightly along the surface, noting by the _feel_ whether or not it takes
hold and also looking with a lens to see if a scratch has been made. Do
not mistake a line of steel, left on a slightly rough surface, for a
true scratch. Frequently on an unpolished girdle of real gem material
the file will leave a streak of steel. Similarly when using test
minerals in accordance with what follows do not mistake a streak of
powder from the yielding test material, for a true scratch in the
material being tested. The safe way is to wipe the spot thus removing
any powder. A true scratch will, of course, persist.

A doublet, being usually constructed of a garnet top and a glass back,
may resist a file at the girdle if the garnet top covers the stone to
the girdle, as is sometimes the case, especially