256 NATURAL HISTORY OF PRECIOUS STONES, &c.
the
real Diamond. But there is one infallible distinction between this
uncoloured gem, and also between the White Topaz, and the true Diamond,
that neither possesses the iridescence always displayed by the latter
when catching the light at a certain angle.*
De Boot states (ii. 32) that he had seen an Oriental Amethyst (i.e., a
purple Sapphire) treated in this way, valued by the Imperial jeweller
at 200 thalers, in consequence of its possessing the true water of the
Diamond, and which could not be distinguished from a real Diamond of
the same size and shape which had cost 18,000 gold pieces. The
engravings on Diamond really done by Birago, Jacopo da Trezzo,
and other artists of the Eenaissance, were often imitated by others,
their contemporaries, either upon this material or the White Topaz.
In
this class of gems the subject-matter, the Precious Corundum,·)· is
extremely capricious in the colours it assumes, from the various
natural influences that may have unequally affected the crystal during
its formation : sometimes the same piece will be blue and red at
opposite ends, each portion quite distinct ; sometimes the colours run
into each other, producing a lilac in their junction ; at other times
the two combine, yet separate when viewed at different angles, so that
the same piece is in one light blue, in another lilac ; or again, the
deepest indigo and perfect whiteness are found in the same crystal, and
so on. A curious variety occurs when the mass is made up of concentric
layers, like the coats of an onion ; such a gem, when polished, is
opalescent, and if skilfully cut, with proper attention to the
arrangement of the layers, will pre-
