IV. Pearly brightness; that which resembles the particular light of a pearl.
V. Silky brightness; that which gives the appearance of silk-stuff.
The
value of a gem depends principally on the beauty of its colour. The
colouring material is generally composed of metallic oxides.
The
wonderful variety of colours in the gems, which more than all others
resemble those of the solar spectrum, gave rise to the diversity of
names which are often given to the same substance. The red corundum is
the oriental ruby; the blue corundum is the oriental sapphire; the
yellow corundum is the oriental topaz. The bluish-green emerald is now
called beryl. Quartz has many names; and more or less value in its
different colours.
There are crystals which not only have a varied gradation of the same colour, but also present three distinct colours.
I
possess a quartz in which the central disc is red, surrounded by a
green zone, ending in a white band. Corundums have been seen in which
shone red, blue and yellow; and we sometimes see the tormaline
variously coloured, watered, and marked with strange figures.
In
some gems the colour differs according to the reflection or
transmission of light. The opal with reflected light is prismatic, and
with transmitted light it is whitish. The tormaline with reflected
light is red, blue, or some other equally bright colour, but with
transmitted light it is olive.
