SPINEL
(_Balas-Ruby_, _Rubicelle_)
Spinel labours under the serious disadvantage of being overshadowed at
almost all points by its opulent and more famous cousins, sapphire and
ruby, and is not so well known as it deserves to be. The only variety
which is valued as a gem is the rose-tinted stone called balas-ruby
(Plate XXVII, Fig. 3), which is very similar to the true ruby in
appearance; they are probably often confused, especially since they
are found in intimate association in nature. Spinels of other colours
are not very attractive to the eye, and are not likely to be in much
demand. Blue spinel (Plate XXVII, Fig. 4) is far from common, but the
shade is inclined to steely-blue, and is much inferior to the superb
tint of the true sapphire. Spinel is very hard and eminently suitable
for a ring-stone, but is seldom large and transparent enough for larger
articles of jewellery.
Spinel is an aluminate of magnesium corresponding to the formula
MgAl_{2}O_{4}, and therefore is closely akin to corundum, alumina, and
chrysoberyl, aluminate of beryllium. The composition may, however,
vary considerably owing to the isomorphous replacement of one element
by another; in particular, ferrous oxide or manganese oxide often
takes the place of some magnesia, and ferric oxide or chromic oxide is
found instead of part of the alumina. When pure, spinel is devoid of
colour, but such stones are exceedingly rare. No doubt chromic oxide is
responsible for the rose-red hue of balas-ruby, and also, when tempered
by ferric oxide, for the orange tint of rubicelle, and manganese is
probably the cause of the peculiar violet colour of almandine-spinel.
It is scarcely possible to define all the shades between blue and red
that may be assumed by spinel. Stones which are rich in iron are known
as pleonaste or ceylonite; they are quite opaque, but are sometimes
used for ornamental wear.
The name of the species comes from a diminutive form of σπῖνος, a
spark, and refers to the fiery red colour of the most valued kind
of spinel. It may be noted that the Latin equivalent of the word,
_carbunculus_, has been applied to the crimson garnet when cut
_en cabochon_. Balas is derived from _Balascia_, the old name for
Badakshan, the district from which the finest stones were brought in
mediæval times.
[Illustration: FIGS. 73, 74.—Spinel Crystals.]
Spinel, like diamond, belongs to the cubic system of crystalline
symmetry, and occurs in beautiful octahedra, or in flat
triangular-shaped plates (Figs. 73, 74) the girdles of which are
cleft at each corner, these plates being really twinned octahedra.
The refraction is, of course, single, and there is therefore no
double refraction or dichroism; this test furnishes the simplest
way of discriminating between the balas and the true ruby. Owing to
isomorphous replacement the value of the refractive index may lie
anywhere between 1·716 and 1·736. The lower values, about 1·720,
correspond to the most transparent red and blue stones; the deep
violet stones have values above 1·730. Spinel possesses little
colour-dispersion, or ‘fire.’ In the same way the values of the
specific gravity, even of the transparent stones, vary between 3·5
and 3·7, but the opaque ceylonite has values as high as 4·1. Spinel
is slightly softer than sapphire and ruby, and has the symbol 8 on
Mohs’s scale, and it is scarcely inferior in lustre to these stones.
Spinel is easily separated from garnet of similar colour by its lower
refractivity. Spinels run from 10s. to £5 a carat, depending on their
colour and quality, and exceptional stones command a higher rate.
Spinel always occurs in close association with corundum. The balas
and the true ruby are mixed together in the limestones of Burma and
Siam. Curiously enough, the spinel despite its lower hardness is found
in the river gravels in perfect crystals, whereas the rubies are
generally waterworn. Fine violet and blue spinels occur in the prolific
gem-gravels of Ceylon. A large waterworn octahedron and a rough mass,
both of a fine red colour, are exhibited in the Mineral Gallery of the
British Museum (Natural History), and a beautiful faceted blue stone is
shown close by.
The enormous red stone, oval in shape, which is set in front of the
English crown, is not a ruby, as it was formerly believed to be,
but a spinel. It was given to the gallant Black Prince by Pedro the
Cruel after the battle of Najera in 1367, and was subsequently worn
by Henry V upon his helmet at the battle of Agincourt. As usual with
Indian-fashioned stones it is pierced through the middle, but the hole
is now hidden by a small stone of similar colour.
The British Regalia also contains the famous stone called the Timur
Ruby or Khiraj-i-Alam (Tribute of the World), which weighs just over
352 carats, and is the largest spinel-ruby known. It is uncut, but
polished. Its history goes back to 1398, when it was captured by the
Amir Timur at Delhi. On the wane of the Tartar empire the stone became
the property of the Shahs of Persia, until it was given by Abbas I to
his friend and ally, the Mogul Emperor, Jehangir. It remained at Delhi
until, on the sack of that city by Nadir Shah in 1739, it, together
with immense booty, including the Koh-i-nor, fell into the hands of
the conqueror. Like the great diamond, it eventually came into the
possession of Runjit Singh at Lahore, and on the annexation of the
Punjab in 1850 passed to the East India Company. It was shown at the
Great Exhibition of 1851, and afterwards presented to Queen Victoria.
Mention has been made above (p. 121) of the blue spinel which is
manufactured in imitation of the true sapphire. The artificial stone is
quite different in tint from the blue spinel found in nature.