NDER the generic name Spinel several minerals are included, as the Spinel Ruby, the Balas Ruby, and the Pleonaste.
Differing
among themselves in colour and other trivial characteristics, they all
agree in possessing approximately the same chemical composition. They are, in fact, aluminates
of magnesia, or compounds of alumina and magnesia, associated with
variable proportions of other metallic oxides, such as those of
chromium and iron, to which the colours are probably due.
Few
minerals enjoy a wider range of colour than the Spinel. Among its
varied tints we may mention carmine, red, reddish-brown, rose-red,
various tints of orange, indigo blue, green, purple, puce, violet, and
even white and yellow Some varieties are opaque and dark-coloured, or
even black, but these have no value as ornamental stones_ Indeed, it is
only the so-called Precious Spinel, which is of use to the
jeweller. By ancient writers the Red Spinels were probably included,
with several other stones, under the general name of Carbunculus.
Precious
Spinels are found either detached, as loose pebbles, or embedded in
granular limestone, or in granite rocks. Jn Burma, Ceylon, and
Badakshan Spinel occurs in well-formed, sharp-angled crystals which are
regular octahedra ; while in many gem sands it is found as rolled
crystals,' accompanied with zircon, garnet, magnetic iron ore, and
other minerals..
The form of Spinel, which is generally that of the regular octahedron, enables it to be readily distinguished
