at's-eye of this is very similar to the quartz
cat's-eye, but a comparison will make the difference so clear that they
could never be mistaken, apart from the fact that the quartz has a
specific gravity considerably lower than the chrysoberyl cat's-eye,
which latter is the true cat's-eye, and the one usually understood when
allusion is made to the stone without any distinguishing prefix, such as
the ruby, sapphire, quartz, etc., cat's eye. It should, however, be
mentioned that this stone is referred to when the names Ceylonese and
Oriental cat's-eye are given, which names are used in the trade as well
as the simple appellation, "cat's eye." One peculiarity of some of these
stones is that the "fire" or "glow" is usually altered in colour by the
colour of the light under which it is seen, the change of colour being
generally the complementary. Thus, a stone which in one light shows red,
in another will be green; the "eye" showing blue in one light will
become orange in another; whilst the yellow of another stone may show a
decided purple or amethyst in a different light.

A good test for this, and indeed most precious stones, is that they
conduct heat more quickly than does glass, and with such rapidity that
on breathing upon a stone the warmth is conducted instantly, so that,
though the stone is dimmed the dimness vanishes at once, whereas with
glass the film of moisture fades but slowly in comparison.

_The Topaz._

The name topaz is derived from the Greek _topazos_, which is the name of
a small island situated in the Gulf of Arabia, from whence the Romans
obtained a mineral which they called topazos and topazion, which mineral
to-day is termed chrysolite. The mineral topaz is found in Cornwall and
in the British Isles generally; also in Siberia, India, South America
and many other localities, some of the finest stones coming from Saxony,
Bohemia, and Brazil, especially the last-named. The cleavage is perfect
and parallel to the basal plane. It crystallises in the 4th (rhombic)
system; in lustre it is vitreous; it is transparent, or ranging from
that to translucent; the streak is white or colourless. Its colour
varies very much--some stones are straw-colour, some are grey, white,
blue, green, and orange. A very favourite colour is the pink, but in
most cases this colour is not natural to the stone, but is the result of
"burning," or "pinking" as the process is called technically, which
process is to raise the temper