weight
it is only worth about half as much as a good Euby. Balas Ruby is worth
about a quarter of the true Euby of the same weight. What was probably
the largest known Spinel of fine quality was one exhibited at the 1862
Exhibition in London; it was cut en cabochon and weighed 197
carats. Another large one is said to have been in the Treasury of St.
Denis, and the shield of the Shah of Persia had a very brilliant Spinel
at its lower point. A famous engraved specimen was one in the Ehodes
Gems carved with a Gorgon's head.
242. Chrysoberyl.
This stone was not the Chrysoberyl of the ancients, which was probably either our Chrysoprase or our Chrysolite.
Chrysoberyl
is a mineral of greenish colour, variously seen as grass-green,
asparagus-green, greenish-white, yellow-green, or even
emerald-green—rarely colourless ; the darker rich green kinds are known
as Alexandrite, they have a columbine-red colour by transmitted light.
Another variety is the Cymophane or true Cat's Eye, which shows a
beautiful blue chatoyancy. Chrysoberyl is rather rare in good
specimens and is highly prized; were it more known its popularity
would most likely be greater still. Where the surfaces are bright it is
often seen to be transparent, but since as a rule it is only found in
water-worn fragments and rolled crystals its more common appearance may
be likened to a piece of glass that has been similarly water-worn ;
some specimens are but sub-translucent.
It
has two optic axes or directions in which light can travel with equal
velocities, and its greatest index of refraction is 1'756 and its
least 1*747; its dispersive power is
