tinct cleavage. Its specific gravity varies from 4.70 to 4.88, according to the specimen and the locality.
This
stone, like some of the others described, has a very wide range of
colour, going through reds, browns, greens, yellows, oranges, whites,
greys, blues from light to indigo, notwithstanding which it is somewhat
difficult to imitate scientifically, though its composition of 38 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
