Stones_, pages 64-71, of Chapter VIII., and various
liquids are there recommended. It is doubtful if the practical gem
dealer would find these methods necessary in most cases. Where large
numbers of many different unknown gems have to be determined it would
pay to prepare, and standardize, and use such solutions.

LESSON VII

LUSTER AND OTHER REFLECTION EFFECTS

By the term _luster_ we refer to the manner and degree in which light is
reflected from the _surface_ of a material. Surfaces of the same
material, but of varying degrees of smoothness would, of course, vary in
the vividness of their luster, but the type of variation that may be
made use of to help distinguish gems, depends upon the character of the
material more than upon the degree of smoothness of its surface. Just as
silk has so typical a luster that we speak of it as silky luster, and
just as pearl has a pearly luster, so certain gems have peculiar and
characteristic luster. The diamond gives us a good example. Most diamond
dealers distinguish between real and imitation diamonds at a glance by
the character of the luster. That is the chief, and perhaps the only
property, that they rely upon for deciding the genuineness of a diamond,
and they are fairly safe in so doing, for, with the exception of certain
artificially decolorized zircons, no gem stone is likely to deceive one
who is familiar with the luster of the diamond. It is not to be denied
that a fine white zircon, when finely cut, may deceive even one who is
familiar with diamonds. The author has fooled many diamond experts with
an especially fine zircon, for the luster of zircon does approach,
though it hardly equals, that of the diamond. Rough zircons are
frequently mistaken for diamonds by diamond prospectors, and even by
pickers in the mines, so that some care should be exercised in any
suspicious case, and one should not then rely solely on the luster.
However, in most cases in the trade there is almost no chance of the
unexpected presence of a zircon and the luster test is usually
sufficient to distinguish the diamond. (Zircons are strongly doubly
refractive, as was said in Lesson III. on Double Refraction, and with a
lens the doubling of the back lines may be seen.)

ADAMANTINE LUSTER. The luster of a diamond is called _adamantine_ (the
adjective uses the Greek name for the stone itself). It is keen and cold
and glittering, having a metallic suggestion. A very large per cent. of
the