But
in the coloured varieties decided traces of other oxides occur, such as
those of chromium and iron in the spinel ruby ; oxide of copper in the
grass green spinel called chlorospinel ; and the protoxide of iron in
the darker and opaque varieties. Some specimens of pleonaste, the
black spinel of Ceylon, have been found to contain over 20 per cent, of
ferrous oxide, the protoxide of iron, which takes the place of its
equivalent of magnesia.
Spinels
in true crystals and very hard have been formed artificially, though
not of fine quality and large size, by several different processes,
such as heating alumina, magnesia, and boracic acid together to a very
high temperature. The vapour of aluminium chloride passed over heated
magnesia also produces spinel crystals, so does the strong heating
together of magnesia and alumina. Some imitation blue spinels or
sapphires have been made by the fusion together of alumina, lime, and a
little cobalt. These ingredients have, however, given a blue glassy
mass softer than true spinel, and merely enclosing here and there a few
minute crystals of what may be termed a " lime spinel," the main mass
being
