70 PRECIOUS STONES.
being
dichroic. Under the microscope imitations are seen to be full of lines
and striae, signs of unequal density and strain. There are also
innumerable rounded cavities or bubbles present, unlike the angular
rents and vesicles in natural stones. An aluminium pencil drawn across
the face of a real stone, such as the Diamond, Sapphire, Ruby or
Emerald, will not leave any mark or " streak," but if drawn across a
glass surface will do so. This test is only applicable to the above
precious stones.
Before
dealing with the production of imitation and artificial precious
stones, for convenience, another distinction in terms may be made, a
distinction between the words "imitation" and "artificial." To many
this may appear unnecessary, the two terms being synonymous, but with
the advance of scientific research the chemist can now make in the
laboratory not "glass imitations," but "real stones artificially," and
identical in composition with those found in Nature. Their artificial
production is quite modern, carrying us back only a generation or two,
but imitation precious stones were known in the middle ages, if not
earlier. They were certainly known to the alchemists, for Saint Thomas
Aquinas mentions imitation Jacinth, Sapphire, Emerald, Topaz, and Ruby.
In the middle of the seventeenth century pastes were not manufactured
according to a different formula for each stone, as had formerly been
the case, but to one general formula, much the same as that in use at
the present day.
One
of the chief difficulties in making a suitable glass- is to combine
hardness with a high index of refraction and dispersive power, for a
glass having the latter properties lacks the former, and " vice versa."
The dispersion of
