the margin at once.)
Such a triplet, if held in the sun, will reflect onto a card two images
in pale or white light, one coming from the top surface of the table and
the other from the top surface of the glass slice within. In other
words, it acts in this respect like a doublet. A true emerald would give
only one such reflection, which would come from the top surface of the
table.

[Illustration: FIG. 8.--EMERALD TRIPLET.]

2. Tourmalines, when green, are usually darker than emeralds and of a
more pronounced yellow green, or they may be of too bluish a green, as
is the case with some of the finest of the green tourmalines from
Maine. Connecticut green tourmaline tends more to the dark yellowish
green, and Ceylon tourmaline to the olive green. The stronger dichroism
of the tourmaline frequently reveals itself to the naked eye, and there
is usually one direction or position in which the color of the stone is
very inferior to its color in the opposite direction or position. Most
tourmalines (except the very lightest shades) must be cut so that the
table of the finished stone lies on the side of the crystal, as, when
cut with the table lying across the crystal (perpendicular to the
principal optical axis) the stones are much too dark to be pretty. Hence
when one turns the cut stone so that he is looking in the direction
which was originally up and down the crystal (the direction of single
refraction and of no dichroism) he gets a glimpse of a less lovely color
than is furnished by the stone in other positions. With a true emerald
no such disparity in the color would appear. There might be a slight
change of shade (as seen by the naked eye), but no trace of an ugly
shade would appear.

By studying many tourmalines and a few emeralds one may acquire an eye
for the differences of color that characterize the two stones, but it is
still necessary to beware of the fine glass imitation and to use the
file and also to look with a high-power glass for any rounding bubbles.
The emerald will never have the latter. The glass imitation frequently
does have them. The sharp jagged flaws and cracks that so often appear
in emerald are likely to appear also in tourmaline as both are brittle
materials. The glass imitations frequently have such flaws put into them
either by pinching or by striking the material. Frequently, too, wisps
of tiny air bubbles are left in the glass imitations in such fashion
that unless one scrutinizes them carefu