eration of its substance under exposure to
atmospheric air; otherwise it is useless for all practical purposes of
adornment or ornamentation.

There are certain other characteristics of these curious minerals which
may be classified briefly, thus:--Some stones owe their beauty to a
wonderful play of colour or fire, due to the action of light, quite
apart from the colour of the stone itself, and of this series the opal
may be taken as a type. In others, this splendid play of colour is
altogether absent, the colour being associated with the stone itself, in
its substance, the charm lying entirely in the superb transparency, the
ruby being taken as an example of this class of stone. Others, again,
have not only colour, but transparency and lustre, as in the coloured
diamonds, whilst the commoner well-known diamonds are extremely rich in
transparency and lustre, the play of light alone showing a considerable
amount of brilliancy and beauty of colour, though the stone itself is
clear. Still others are opaque, or semi-opaque, or practically free from
play of light and from lustre, owing their value and beauty entirely to
their richness of colour.

In all cases the value of the stone cannot be appreciated fully till the
gem is separated from its matrix and polished, and in some cases, such
as in that of the diamond, cut in variously shaped facets, on and
amongst which the light rays have power to play; other stones, such as
the opal, turquoise and the like, are cut or ground in flat,
dome-shaped, or other form, and then merely polished. It frequently
happens that only a small portion of even a large stone is of supreme
value or purity, the cutter often retaining as his perquisite the
smaller pieces and waste. These, if too small for setting, are ground
into powder and used to cut and polish other stones.

Broadly speaking, the greatest claim which a stone can possess in order
to be classed as precious is its rarity. To this may be added public
opinion, which is led for better or worse by the fashion of the moment.
For if the comparatively common amethyst should chance to be made
extraordinarily conspicuous by some society leader, it would at once
step from its humbler position as semi-precious, and rise to the nobler
classification of a truly precious stone, by reason of the demand
created for it, which would, in all probability, absorb the available
stock to rarity; and this despite the more entrancing beauty of the now
ra