make, and everyone who deals in colored stones should own and use one.

Not all stones that are doubly refracting exhibit dichroism. White
stones of course cannot exhibit it even though doubly refracting, and
some colored stones, though strongly doubly refracting, do not exhibit
any noticeable dichroism. The zircon, for example, is strongly doubly
refracting, but shows hardly any dichroism.

The test is most useful for emerald, ruby, sapphire, tourmaline,
kunzite and alexandrite, all of which show marked dichroism.

It is of little use to give here the twin colors in each case as the
shades differ with different specimens, according to their depth and
type of color. The deeper tinted stones of any species show the effect
more markedly than the lighter ones.

The method is rapid and easy--it can be applied to mounted stones as
well as to loose ones, and it cannot injure a stone. The student should,
if possible, obtain the use of a dichroscope and practice with it on all
sorts of stones. He should especially become expert in distinguishing
between rubies, sapphires, and emeralds, and their imitations. The only
imitation (scientific rubies and sapphires are not here classed as
imitations), which is at all likely to deceive one who knows how to use
the dichroscope is the emerald triplet, made with real (but pale) beryl
above and below, with a thin strip of green glass between. As beryl is
doubly refracting to a small degree, and dichroic, one might perhaps be
deceived by such an imitation if not careful. However, the amount of
dichroism would be less in such a case than in a true emerald of as deep
a color.

Those who wish to study further the subject of dichroism should see
_Gem-Stones_, by G. F. Herbert-Smith, Chapter VII., pp. 53-59, or see _A
Handbook of Precious Stones_, by M. D. Rothschild, Putnam's, pp. 14-16.

LESSON V

SPECIFIC GRAVITY

The properties so far considered as serving to distinguish precious
stones have all depended upon the behavior of the material toward light.

These properties were considered first because they afford, to those
acquainted with their use, very rapid and sure means of classifying
precious stones.

DENSITY OF MINERALS. We will next consider an equally certain test,
which, however, requires rather more time, apparatus, and skill to
apply.

Each kind of precious stone has its own _density_. That is, if pieces of
_different_ stones were taken all of the same size, t