the
second of a violet rose, or a vinegar rose; but there is no absolute
rule for their colour, since Pegu furnishes white and violet-white
spinels; and specimens have been brought from Aker in Sudermania which
are of a bluish gray.
Spinels
are brought also from Ceylon and other oriental countries; everywhere
they are found in the beds of torrents, in the midst of alluvial
deposits.
The
primitive form of the crystals of spinel ruby is octahedral, like that
of the diamond; this characteristic suffices to distinguish
immediately the spinel or balas ruby from the oriental ruby, since the
crystals of the latter present the form of six-sided prisms.
The
composition of the spinel ruby and balas ruby differ essentially from
that of the oriental ruby, the latter being a corundum formed nearly
exclusively of alumina, while in the former only 70 parts in
100 are alumina, and the remainder chiefly magnesia. The colour,
moreover, is in part due to the oxide of chromium, a substance of which
the oriental ruby does not contain the slightest trace.
In
a scientific point of view the balas ruby does not differ from the
spinel; and many special works confound the two completely. But in
commerce the stone called Balas has a value very much below that of the
spinel.
