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PRECIOUS STONES. 167
the
colour is the segregation of the zeolitic matter in the Cachalong
hands, which has the effect of leaving these hands quite translucent in
part or in whole. Again, the Opal bands, which seem to have a tendency
to lose their water, may have passed into ordinary Chalcedony, only
showing their original character by their evidence of deposition under
the influence of gravity alone, surface tension not having affected
them apparently. Still further forms of colouring are due to weathering
of the Agates, and, as one might expect, the change is seen to occur
first on the outer, or older, layers of the Agate, since they are most
exposed; the commonest change is for a red coloured Agate to be
bleached to a chalky white, either throughout or on the outermost
layers. Cracks are found developed from the same exposure, and through
these arises another form of Agate known as Mocha, by the infiltration
along the cracks of solutions of manganese, and the deposit within the
cracks of Pyrolusite in dendritic forms. In some cases, the whole of
the Agate within the " skin," or perhaps only the most recently formed
part, may be filled with crystalline or even crystallised Quartz,
either as Rock Crystal, Smoky Quartz, or Amethyst, thus giving a
colourless, a smoky or a violet interior to the stone; the Agate in
such a case may consist of anything from a mere hollow shell to a solid
mass. The Quartz in these instances may contain needles or scales of
Gothite; more rarely the interior of the Agate may be pink from the
presence of Dolomite.
The lustre varies with that of the different constituents, as do the diaphaneity and other physical characters.
The origin of Agate has been considered in dealing with the origin of precious stones in general.
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