has a peridot (or chrysolite) by the
luster alone, but it will pay to spend some spare time in studying the
luster of the various stones.

A true, or "precious" topaz, for example, may be compared with a yellow
quartz-topaz, and owing to the greater hardness of the true topaz, it
will be noted that it has a slightly keener luster than the other stone,
although both have vitreous luster. Similarly the corundum gems (ruby
and sapphire), being even harder than true topaz, take a splendid
surface finish and have a very keen vitreous luster.

Turquoise has a dull waxy luster, due to its slight hardness. Malachite,
although soft, has, perhaps because of its opacity, a keen and sometimes
almost metallic luster.

One may note the luster rapidly, without apparatus and without damage to
the stone. We thus have a test which, while it is not conclusive except
in a very few cases, will supplement and serve to confirm other tests,
or perhaps, if used at first, will suggest what other tests to apply.

Another optical effect that serves to distinguish some stones depends
upon the reflection of light from within the material due to a certain
lack of homogeneity in the substance.

CAUSE OF COLOR IN THE OPAL. Thus the opal is distinguished by the
prismatic colors that emerge from it owing to the effect of thin layers
of material of slightly different density, and hence of different
refractive index from the rest of the material. These thin films act
much as do soap-bubble films, to interfere with light of certain wave
lengths, but to reflect certain other wave lengths and hence certain
colors.

Again, in some sapphires and rubies are found minute, probably hollow,
tube-like cavities, arranged in three sets in the same positions as the
transverse axes of the hexagonal crystal. The surfaces of these tubes
reflect light so as to produce a six-pointed star effect, especially
when the stone is properly cut to a high, round cabochon form, whose
base is parallel to the successive layers of tubes.

STARSTONES, MOONSTONES, CAT'S-EYES. In the moonstone we have another
sort of effect, this time due to the presence of hosts of small twin
crystal layers that reflect light so as to produce a sort of
moonlight-on-the-water appearance _within_ the stone when the latter is
properly cut, with the layers of twin crystals parallel to its base.
Ceylon-cut moonstones are frequently cut to save weight, and may have to
be recut to properly place the laye