t in water = 1.00 carat

Weight of stone 4.02
Specific gravity = --------------- = ---- = 4.02
Loss in water 1.00

Here the specific gravity, 4.02 would indicate some corundum gem (ruby
or sapphire), and the other characters would indicate at once which it
was.

The student who means to master the use of the two methods given in
Lessons V. and VI. should proceed to practice them with stones of known
specific gravities until he can at least get the correct result to the
first decimal place. It is not to be expected that accurate results can
be had in the second decimal place, with the balances usually available
to jewelers. When the learner can determine specific gravities with some
certainty he should then try unknown gems.

The specific gravity method is of especial value in distinguishing
between the various colorless stones, as, for example, quartz crystal,
true white topaz, white sapphire, white or colorless beryl, etc. These
are all doubly refractive, have no color, and hence no dichroism, and
unless one has a refractometer to get the refractive index, they are
difficult to distinguish. The specific gravities are very different,
however, and readily serve to distinguish them. It should be added that
the synthetic stones show the same specific gravities as their natural
counterparts, so that this test does not serve to detect them.

Where many gems are to be handled and separated by specific gravity
determinations, perhaps the best way to do so is to have several liquids
of known specific gravity and to see what stones will float and what
ones will sink in the liquids. Methylene iodide is a heavy liquid (sp.
g. 3.32), on which a "quartz-topaz," for example, sp. g. 2.66, would
float, but a true topaz, sp. g. 3.53, would sink in it. By diluting
methylene iodide with benzol (sp. g. 0.88) any specific gravity that is
desired may be had (between the two limits 0.88 and 3.32). Specimens of
known specific gravity are used with such liquids and their behavior (as
to whether they sink or float, or remain suspended in the liquid,)
indicates the specific gravity of the liquid. An unknown stone may then
be used and its behavior noted and compared with that of a known
specimen, whereby one can easily find out whether the unknown is heavier
or lighter than the known sample.

An excellent account of the detail of this method is given in G. F.
Herbert-Smith's _Gem-