44 GEMS AND PRECIOUS STONES.
The variety Alexandrite is
a dark-green by daylight, but changes to a raspberry-red hue by
artificial light; this change of colour affords a good illustration of
dichroism. By a system of twinning, this variety presents the
appearance of an obtuse hexagonal pyramid. Some very fine specimens of
alexandrite, free from flaws, and of large size, have been at different
times sent to Europe from Ceylon, weighing in some instances as much as
GO carats. Its discovery in the Urals is of recent date, although the
original stone came from that locality.
The name Cymophane, meaning
to appear like a wave of light, is given to those specimens of
chrysoberyl that appear to enclose rays of light, a phenomenon no doubt
the result of internal reflections. Some good specimens of this variety
are in the collection at South Kensington Museum.
The most valuable variety of the chrysoberyl, however, is the cat's-eye (also
known as cymophane), a rare gem, and not to be confounded with the
quartz " cat's-eye," a stone of little commercial value. It is
distinguished by its remarkable appearance when a ray of light acts
upon it in certain directions. This gem is generally cut en cahochon, and
when well polished shows a line like a silver wire; these lines are
supposed to be caused by the minute internal striations of the
crystals. It was called in ancient times " oculus solis "; this eye of
the sun is at the present time a favourite in China, and commands a
high price there. In India it is greatly prized on account of its
supposed power against witchcraft, and it has been stated that this
stone is the last that a Cingalese will part with. In India the value
rises as the bright lines increase in number; these lines of light are
there called "betas."
The
colour of some of these stones is quite dark, the fancy colour in
America being almost black ; the white lines show more distinctly when
the ground colour is dark. This gem has become more fashionable of late
years, especially in America.
In
perfectly cut stones the line should run evenly from end to end, and be
in the middle of the stone, should be well defined, not broad, and the
ground colour sufficiently dark to contrast well with the line.
The
value of good " cat's eyes " appears to be changeable, specimens of
good quality, fit for rings, being valued at from £10 to £100, and Mr.
Streeter states that several large specimens are on the market worth
upwards of £1,000 each.
The
value and importance of good specimens of the true " cat's-eyes" or
chrysoberyl, and the small value of the quartz " cat's-eye," make it
necessary that the distinguishing characteristics should be well
marked. The hardness of the quartz variety is only 7, and the specific
gravity is but 2'6, so that the determination is easy ; besides the ray
of light in the quartz is dull, while that of the true "cat's-eye " is
iridescent, and the difference in brilliancy is quite marked, the
quartz variety being quite dull in comparison. A large specimen of the
true " cat's-eye," measuring 5 inches in length, is in the Vienna
collection.
A very fine specimen from the Hope collection is in the British Museum.
There
is a fibrous variety of quartz that has been of late years found in
South Africa ; it has been obtained of large size, but specimens of
this stone are not likely to be mistaken for the true " cat's-eye," so
that it will be unnecessary to consider it here. The most common
mistake is that the chrysoberyl is frequently called chrysolite, and
mistaken for that gem, but the hardness of chrysolite is but little
more than six, and its specific gravity is also less than that of the
chrysoberyl.
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