crowd
of stones, practically identical in composition, but presenting great
diversity in colour and optical properties. All these varieties belong
however to the mineral species corundum, the French corindon, and
consist of crystallised alumina, the oxide of the metal aluminium. From
the mineralogical, or rather from the physical point of view, the
colour of these stones is of no account, while chemistry has not as yet
succeeded in discovering much concerning the causes of the variations
of colour which determine the very different values set upon different
specimens of corundum. That there are small quantities of magnesia,
oxide of iron and silica in rubies and sapphires of all hues, has been
ascertained, but this fact does not furnish the clue to the cause of
the blue of the sapphire or of the red of the ruby. That certain
chromium compounds impart a red hue to certain artificial preparations,
both crystallised and vitreous, of alumina, will not count for much in
the absence of proof that all rubies contain chromium. That iron is the
cause of the dark colour of emery and other impure corundum is,
however, certain : indeed some specimens of emery contain half their
weight of iron oxide.
Coloured
corundums, when strongly heated, generally change their hue, pale blue
and pale yellow stones becoming colourless, and violet stones retaining
only the red constituent of their original colour. In Ceylon the native
dealers frequently offer for sale specimens exhibiting a beautiful pink
or rose colour which is not natural, but has been prcduced by " firing
" inferior corundums of the purple variety or oriental amethyst.
Corundum
always occurs in crystals or is at least crystalline ; the forms are
six-sided prisms or pyramids belonging to the hexagonal (rhombohedral)
system. The lustre is vitreous except on the basal planes which are
often pearly. The six-rayed star seen in many cloudy sapphires and
rubies, especially when cut en cabochon with the summit of the
curved surface lying in the direction of the principal axis of the
prism, is due to the peculiar intimate structure
