INTRODUCTORY 3
quirements
:—It must withstand the action of light without deterioration of its
beauty, lustre, or substance, and it must be of sufficient hardness to
retain its form, purity and lustre under the actions of warmth,
reasonable wear, and the dust which falls upon it during use ; it must
not be subject to chemical change, decomposition, disintegration, or
other alteration of its substance underexposure 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
