Agate is a form of the common mineral quartz. From other forms of that
mineral it differs in being made up of minute layers and in being
variegated in color. The colors may appear in the form of bands or
clouds. The banded agates appear to be made up of parallel layers,
sometimes straight, but more often wavy or curved in outline. These
layers or bands differ in color from one another, exhibiting shades of
white, gray, blue, yellow, red, brown or black. To the naked eye they
appear to vary in width from the finest lines to a width of a quarter of
an inch or more. In reality, all the bands visible to the naked eye are
made up of finer ones, to be seen only with the microscope. Thus in a
single inch of thickness of agate Sir David Brewster, using the
microscope, counted seventeen thousand and fifty layers. Besides
differing in color, the layers differ in transparency and porosity, and
these properties add to the variegated appearance of the agate.
On account of their beauties of color and outline, agates have been
known and prized from the earliest times. They are mentioned by many of
the ancient Greek writers, and the name agate is a corruption of the
name Achates, a river in Sicily, whence the first stones of this kind
used by the Greeks were obtained. This and neighboring localities
continued to be the source of supply until the fifteenth century, when
agates were found to occur in large quantities near Oberstein and Idar
on the banks of the river Nahe, in the duchy of Oldenburg.
The industry of cutting and polishing the agates on a large scale was
soon established there, and these places are to this day the center of
the agate industry. The agates used most extensively at the present time
are not, however, those found about Oberstein, but come from a region
about one hundred miles in length extending from the Province of Rio
Grande do Sul, of Southern Brazil, into Northern Uruguay.
The agates in this region, first discovered in 1827, so surpass in size
and beauty those from any other known locality, that they form at the
present time almost the only source of supply. They are shipped in large
quantities as ballast to Oberstein and Idar, and here the work of
cutting, polishing and coloring them is performed. The discovery that
the attractiveness of agates could be enhanced by artificial coloring
was made about the beginning of the nineteenth century. The natural
colors are rarely of a high order, being often only variations of white
and gray or dull yellows and reds. Through the difference of porosity of
the different layers, however, and the consequent different absorption
of coloring ingredients, methods of artificial coloring can be employed,
which produce lasting and pleasing effects. Most agate used for
ornamental purposes at the present time is therefore artificially
colored.
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 moss
agates of Colorado and other localities in the Rocky Mountains are,
however, equal to anything in the world.
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 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.
The beautiful moss-like inclusions seen in the moss agates are due to a
partial crystallization of oxide of manganese or iron contained in the
waters. The particles of oxide in these cases arrange themselves in
arborescent forms, just as do the particles of frost crystallizing on a
window pane.
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 use
for large marbles was once quite common, but glass marbles of the same
size and still called “agates” are now generally substituted. In fine
mechanical work, such as bearings for delicate instruments and in tools
for polishing and grinding, agate is still extensively used.
Oliver Cummings Farrington.