but
it will be considered here chiefly with regard to the red varieties
approximating the colour of the ruby. Spinel is practically a magnesium
aluminate, consisting of alumina, 71.8%, and magnesia, 28.2%. The chief
red shades are: deep red, Siam ruby and spinel ruby; rose red, balas
ruby; yellow or orange red, rubicelle; violet red, almandine ruby. The
native name in India for spinel is " pomegranate." A slight knowledge
of mineralogy should suffice to distinguish the corundum ruby from its
spinel distant relative, for the latter is less hard and of lower
specific gravity, and different in crystallisation. Spinel is of about
the hardness of topaz, or 8 in the Mohs scale, and its specific
gravity is about 3.6. It crystallises in the isometric system and
usually appears in the form of octahedrons. It is singly refracting,
corundum doubly. Spinel is infusible before the blowpipe, but heating
it will cause it to undergo several changes of colour, ultimately
returning to its original hue, so that it might be termed the chameleon
of gem minerals. Without any design to substitute spinel for corundum
rubies, spinel has its own deserved value, and its beauty and intrinsic
worth deserve for it an inclusion in the company of the high-class gems.
