ning
parallel to the meeting of the base and prism. Another common habit of
Corundum is a stout hexagonal prism terminated by the basal plane ;
this type is often met with in Ruby, in which also the rhombohedral
habit is seen most frequently.
Chemical composition. Corundum is pure alumina, AL2O3
, the oxide of Aluminium. Traces of ferric oxide, silica and oxide of
chromium are found in even the purer varieties often, and it is
supposed (though it is by no means certain) that these substances,
slightly varying in amount and proportion, give to the Ruby, the
Sapphire, and the other precious forms of Corundum their beautiful
colours on which the value so largely depends. The less pure forms of
Corundum contain large amounts of iron, especially as Magnetite and
Haematite.
The
different forms of Corundum do not behave alike under the influence of
heat. Thus a Ruby after being heated retains its colour, hence the
colour certainly is not organic in nature; Sapphire, on the other hand,
has its colour discharged in the majority of cases by heating, and from
this it has been suggested that organic matter may account for the
beautiful blue. However, the prevailing opinion is that both owe their
beauty to small quantities of chromium and possibly of iron also. Ruby
may with care be heated to very high temperatures; it turns a dirty
grey colour when very hot, but on cooling it again turns to red,
passing through white and green as it cools.
In
considering the origin of these gems, it may be well to deal with their
distribution at the same time, merely noticing in passing that Corundum
seems in the great majority of
