in the examination of the stone was to clean it and to give it a
careful examination with a ten-power lens. (An aplanatic triplet will
be found best for this purpose.)

COLOR. The color was, of course, the most obvious property, but, as has
already been said, color is not to be relied upon in all cases. In this
case the color was a good emerald green but a bit bluer than the finest
grass green. A very fine Maine tourmaline might approach this stone in
color, so it became necessary to consider this possibility. A glass
imitation, too, might have a color equal or superior to this.

IMPERFECTIONS. While noting the color, the imperfections of the stone
claimed attention. They consisted mainly of minute jagged cracks of the
character peculiar to brittle materials such as both emerald and
tourmaline. So far it will be noted either of the above minerals might
have furnished the lady's gem. As glass can be artificially crackled to
produce similar flaws the stone might have been only an imitation as far
as anything yet learned about it goes.

FILE TEST. The next step was to test its hardness by gently applying a
very fine file to an exposed point at one corner of the girdle. The file
slipped on the material as a skate slips on ice. Evidently we did not
have to do with a glass imitation.

REFRACTION. Knowing now that we had a true hard mineral, it remained to
be determined what mineral it was. On holding the stone in direct
sunlight and reflecting the light onto a white card it was seen at once
that the material was doubly refracting, for a series of _double_ images
of the back facets appeared. These double images might have been
produced by tourmaline as well as by emerald. (Not however by glass
which is singly refracting.) If a direct reading refractometer had been
available the matter could have been settled at once by reading the
refractive indices of the material, for tourmaline and emerald have not
only different refractive indices but have double refraction to
different degrees. Such an instrument was not available at the time and
will hardly be available to most of those who are studying this lesson,
so we can go on with our account of the further testing of the green
stone.

HARDNESS. A test upon the surface of a quartz crystal showed that the
stone was harder than quartz (but so is tourmaline). A true topaz
crystal was too hard for the ring stone, whose edge slipped over the
smooth topaz surface. The green stone wa