it
may be with other substances; but it now and then occurs in absolute
purity; and it is always possible to extract pure alumina from any kind
of clay.
If
we ask now what is the composition of the precious stones whose names
figure at the head of this chapter, we are answered—they are formed of
alumina nearly pure. Besides this they contain only some faint traces
of foreign matters, generally of the oxide of iron.
Notwithstanding
the minute quantities, these foreign matters are very important,
because it is to their union with alumina that the precious stones we
are considering owe their remarkable colour, and consequently a great
part of their commercial value.
But
if the ruby, the sapphire, and kindred gems, are formed almost
exclusively of alumina, we must hasten to add that this alumina is crystallized, for
in this fact is comprehended the cause of the enorÂmous distance which
separates the alumina of the soil around us from the alumina of which
precious stones are composed.
CORUNDUM.
Modern mineralogists have given the single name corundum to all the minerals consisting of crystal-
