But
the spinel owns other hues which labour under no such disadvantages.
There is a brilliant aurora red, and a whole suite of passage hues,
between indigo and puce, which stand alone for curious beauty. And to
these may be added steel-grey, slate and sky-blue, without exhausting
the colours offered by the spinel.
The
red varieties of the spinel are generally spoken of as " spinel ruby "
and " balas ruby," those designated bv the latter name being inferior
in colour and brilliancy, and less like the true ruby ; but, chemically
and physically, there is no sharp distinction between diem. " Rubicelle
" is a term applied to orange and flame-coloured spinels, those that
are violet and purple being called " almandine " spinels.
Spinels
fit for use in jewellery come from many localities. Of these the chief
are in Burma, Siam, and Ceylon ; also in the United States, in New
York, and New Jersey, but the fine large red spinels come exclusively
from India ; several of these, weighing, when properly cut, as much as
25, 72, 81 carats, have recently found their way to Europe. One
rose-coloured specimen weighed as much as 150 carats, but being too
pale was sold for no more than £80. An unusually large and fine crimson
spinel or spinel-ruby, weighing 49-1/64 carats, from India, proved to
have the specific gravity 3.582, while its index of refraction was—for
the red ray, 1.711, for the yellow ray, 1.714 and for the blue ray,
1.719. Its hardness scarcely exceeded that of the red garnet, but the
tests of specific gravity and refraction sufficed to show that it was a
true spinel. A spinel ruby of rich colour is seen probably to the best
advantage when step-cut ; the paler hues often look well brilliant-cut.
A border of small diamonds enchances the colour and beauty of a spinel.
The
spinel crystallizes in the cubic or monometric system, a regular
octahedron being its most common form. Some of the natural crystals of
spinel are so perfect in shape and polish that they are quite fit for
ornamental mounting without further preparation. The hardness of this
gem is 8, while its specific gravity
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