e outside of the bottle perfectly dry before
each weighing. The bottle should never be held in the warm hands, or it
will act like a thermometer and expand the water up the narrow tube in
the stopper, thus leading to error. A handkerchief may be used to grasp
the bottle.

TABLE OF SPECIFIC GRAVITIES OF THE PRINCIPAL GEM MATERIALS

Beryl (Emerald) 2.74
Chrysoberyl (Alexandrite) 3.73
Corundum (Ruby, sapphire, "Oriental topaz") 4.03
Diamond 3.52
Garnet (Pyrope) 3.78
" (Hessonite) 3.61
" (Demantoid, known in the trade as "Olivine") 3.84
" (Almandite) 4.05
Opal 2.15
Peridot 3.40
Quartz (Amethyst, common topaz) 2.66
Spinel (Rubicelle, Balas ruby) 3.60
Spodumene (Kunzite) 3.18
Topaz (precious) 3.53
Tourmaline 3.10
Turquoise 2.82
Zircon, lighter variety 4.20
" heavier variety 4.69

For a more complete and scientific discussion of specific gravity
determination see _Gem-Stones_, by G. F. Herbert-Smith, Chapter VIII.,
pp. 63-77; or see, _A Handbook of Precious Stones_, by M. D. Rothschild,
pp. 21-27, for an excellent account with illustrations; or see any
physics text-book.

LESSON VI

SPECIFIC GRAVITY DETERMINATIONS

WEIGHING A GEM IN WATER. In the previous lesson it was seen that the
identity of a precious stone may be found by determining its specific
gravity, which is a number that tells how much heavier the material is
than a like volume of water. It was not explained, however, how one
would proceed to get the specific gravity of a stone too large to go in
the neck of a specific gravity bottle. In the latter case we resort to
another method of finding how much a like volume of water weighs. If the
stone, instead of being dropped into a perfectly full bottle of water
(which then overflows), be dropped into a partly filled glass or small
be