Spinel
is a mineral somewhat similar to ruby, but as spinel gems are quite
different from true rubies, so the mineral spinel is quite distinct
from the mineral of the true ruby, which is corundum. In
composition spinel is almost wholly a magnesia aluminate, and in
hardness is much softer than the ruby, being No. 8, or no harder than
the topaz, while the ruby is No. 9.
Spinel occurs in various colors, but the most valuable color is red, called the spinel ruby. From
the fact that the spinel so closely approaches the ruby in appearance,
it has often been confounded with it, and in cases of fraud spinel is
sometimes sold in place of the corundum ruby. In its own sphere,
however, the spinel is a beautiful gem, and while never found in very
large sizes, has yet been so highly prized that it is found amongst the
crown jewels of several nations. The ruby of the "Black Prince," which
is shown among the crown jewels of England, is a notable example of
spinel.
