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334 THE CURIOUS LORE OP PRECIOUS STONES
star-sapphire
and the star-ruby, all these being considered to bring good fortune to
the wearer. A splendid s-tar-sapphire is in the hilt of the sword
presented as an Easter gift to King Constantine of Greece, then Prince
Constantine, by the Greeks of America, on Easter Day 1913.12
This ornate and beautiful sword was made by Tiffany & Co. Then
there is the alexandrite cat's-eye which, in addition to its chatoyant
effect, changes from green to red, showing its natural color by day and
glowing with a ruddy hue by artificial light. The cat's-eye effect here
is caused by a twinning of the crystal ; that is, when the gem is cut,
with a dome, across the twinning line, this shows itself as a smooth
band of white light, with a translucent or transparent space at one
side, the line varying in sharpness and in breadth as the illumination
becomes more intense. If the light is very bright, the line is no wider
than the thinnest possible silver or platinum wire.
The
quartz cat's-eye, less distinct than the chryso-beryl cat's-eye, is
also found in the East, and possesses the property that when cut
straight across, an apparent striation in the stone produces the
cat's-eye effect, but the material is not so rich or brilliant nor is
the gem as beautiful as is the true cat's-eye. The alexandrite variety
of chrysoberyl is colored by chromium and is dichroitic, appearing
green when viewed in one direction and red in another ; in artificial
light, however, the green color is lost and the red alone becomes
apparent.
The
moonstone, with its moonlike, silvery-white light, changes on the
surface as the light varies. This is due to a chatoyancy produced by a
reflection caused by certain cleavage planes present in feldspar of the
variety to which the moonstone belongs.
K The star-sapphire has already been described on pp. 106, 107.
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