hould be resorted to only when there remains a reasonable doubt after
other tests have been applied.

Now for our final example. A red stone, cut in the form of a pear-shaped
brilliant, was submitted to the writer for determination. It had been
acquired by an American gentleman in Japan from an East Indian who was
in financial straits. Along with it, as security for a loan, the
American obtained a number of smaller red stones, a bluish stone, and a
larger red stone. The red stones were all supposed to be rubies. On
examination of the larger red stone with a lens it was at once noted
that the internal structure was that of _scientific ruby_.

TESTING OTHER STONES. Somewhat dashed by the announcement of this
discovery the owner began to fear that all his gems were false.
Examination of the small red stones showed abundance of "silk," a
peculiar fibrous appearance within the stone caused by its internal
structure. The fibers were _straight_ and _parallel_, not _curved_ and
_parallel_ as in synthetic ruby. Tiny bubbles of angular shape also
indicated that the small stones were natural rubies. They exhibited
dichroism and scratched topaz and it was therefore decided that they at
least were genuine.

The pear-shaped brilliant which was first mentioned was of a peculiar,
slightly yellowish, red color. It was very pellucid and free from any
striae either of the straight or curved types. It had in fact no flaws
except a rather large nick on one of the back surfaces near the girdle.
This was not in evidence from the front of the stone and had evidently
been left by the Oriental gem cutter to avoid loss in weight while
cutting the stone.

The peculiar yellowish character of the red color led us to suspect ruby
spinel. The stone was therefore inspected with the dichroscope and
found to possess no dichroism. The sunlight-card test, too, showed that
the stone was singly refracting.

A test of the hardness showed that the material barely scratched topaz,
but was attacked by sapphire. It was therefore judged to be a red
spinel.

The large bluish stone which the gentleman acquired with the red stones
proved to be iolite, sometimes called cordierite or water-sapphire
(_Saphir d'eau_), a stone seldom seen in this country. It had marked
dichroism--showing a smoky blue color in one direction and a yellowish
white in another. The difference was so marked as to be easily seen
without the dichroscope.

LESSON XVII

SUITABILITY