whites,
greys, blues from light to indigo, notwithstanding which it is somewhat
difficult to imitate scientifically, though its composition of 33 per
cent. of silica with 67 per cent. of zirconia (the oxide of zirconium),
is practically all it contains, apart from the colouring matter, such as
the metallic oxides of iron, uranium, etc. Its hardness is 7-1/2,
consequently it is untouched by a file, and so far, if one or perhaps
two of the three qualities of colour, hardness, and specific gravity,
are obtained in a chemically made zircon, the third is wanting. Under
the blowpipe, zircons are infusible, but the coloured stones when heated
strongly become heavier, and as they are contracting, their colour
fades, sometimes entirely, which changes are permanent, so that as they
possess the adamantine lustre, they are occasionally cut like a diamond,
and used as such, though their deficiency in fire and hardness, and
their high specific gravity, make them readily distinguishable from the
diamond.

On exposure to light the coloured zircon becomes more or less
decoloured; especially is this so in sunlight, for when the direct rays
of the sun fall upon it, the colours fade, and for a moment or two
occasional phosphorescence follows, as is the case when the stone is
warmed or heated in a dark room. The stone appears to be very
susceptible to brilliant light-rays, and in certain specimens which were
split for testing, one half of each being kept excluded from light for
purposes of comparison, it was found that sunshine affected them most;
then brilliant acetylene gas, which was more effective still when tinted
yellow by being passed through yellow glass. The electric arc was not so
effective, but the electric light of the mercury-vapour lamp, though
causing little change at the first, after a few hours' exposure rapidly
bleached certain of the colours, whilst having no effect on others. Coal
gas with incandescent fibre mantle was slightly effective, whilst the
coal-gas, burned direct through an ordinary burner, affected very few of
the colours, even after twenty-four hours' exposure at a distance of
three feet. In all these cases, though the colours were slightly
improved by the stones being kept for a time in the dark, they failed to
recover their original strength, showing permanent loss of colour.

_The Silicates._

The chief of these are the garnets, crystallising in the cubic system,
and anhydrous. The garnet is usually i