pleasing
effects. Most agate used for ornamental purposes at the present time
is, therefore, artificially colored. The method of coloring is to boil
the stone in honey for a number of days, or even weeks, according to
the porosity of the agate and the color desired, then to immerse it in
hot sulphuric acid. The acid chars to a brown or black the carbon of
the honey which has been absorbed by the stone. Various coloring
ingredients, such as oxides of iron, salts of nickel, Prussian blue,
etc., may be added to the liquids employed at some stage of the
process, and thus different colors be obtained.
Agates
of considerable beauty, though not of great size, are found in many
places in the United States. Those of Agate Bay, Lake Superior, have
rich colors, and make attractive charms and' other ornaments. Agates
are found in the beds of many streams in Colorado, Montana, and other
regions of the Rocky Mountains. They occur all along the Mississippi
River, especially in Minnesota, also along the Fox River, Illinois, in
the trap rocks along the Connecticut River, and on the coast of
California. While many of these agates are of great beauty, their use
and sale is not likely to be anything more than local, since the
Brazilian agates can be supplied so cheaply from Germany.
The
layered structure of agates is due to successive depositions of silica
by water flowing through cavities in rocks. Rising and falling
alternately through the rocks the water leaves a mark of each advance
or retreat in the form of an additional layer deposited upon the
interior walls of the cavity. Agates, therefore, grow from the outside
inward. The process may go on until the cavity is entirely filled or
may cease at any time. If the cavity is small and nearly circular, and
becomes entirely filled, the kind of agate known as "eye-agate" is
produced. If water remains in the cavity for some time crystals, such
as are sometimes seen, will be formed. The nodule of silica or agate
formed by the percolating waters is harder and more resistant than the
surrounding rock. Hence it remains after the surrounding rock has been
worn away. We can thus understand why agates should be found, as they
usually are, on sea or lake beaches, or in the beds of streams.
The
different colors seen in the natural agates are produced by traces of
organic matter or of oxides of iron, manganese or titanium contained in
the waters which formed them.
Agates
are not used as extensively as they once were for ornamental purposes.
In the years of 1848-50 agate jewelry was very fashionable, and was
extensively worn. At the present time, however, the principal use of
agate in jewelry is for breastpins and watch-charms. For ornamental
purposes it is used in pen-holders, knife-handles, and vases. Its
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