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PRECIOUS STONES.
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THIRD SECTION. AGATE.
The
agate, unlike other precious stones, very rarely occurs in veins; it is
almost always in the state of concretions; sometimes in the form of
geodes or balls. Occasionally there is found in the side of one of
these balls a sort of funnel through which the silicious matter was
introduced.
Sometimes
the gelatinous silica has been abundant enough to give rise to
homogeneous deposits of a certain depth; the stone in that case is of
uniform colour; but often the deposits are in very thin layers, and of
different shades of colour; often, too, they are moulded by the
cavities of the body which forms their support, and take from its
irregularities all sorts of dispositions with very variable shadings.
In
cutting a section across a stone of this category, extremely different
effects are obtained by following different directions. The varied
zones and colours of the stone produce, too, infinite varieties; and
descriptive names have been bestowed upon agates, according to these
changes, as rainbow, cloud, moss, star, ruin, landscape, fortification
agate, &c. The differences between all these varieties are
extremely slight in a physical or chemical point of view.
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