is called banded, or ribbon agate ; when the colors are sharply
defined, it is onyx-agate ; when the stripes converge towards a
centre, it is eye-agate ; if divers colors are displayed, it becomes
iris or rainbow-agate ; when it has the semblance of moss enclosed, it is moclia-stone or moss-agate ; and dendritic, when
foliage and trees are simulated, though moss-agate and dendritic are generally classed as one variety. These vegetable
representations are supposed to have been produced in water
by some metallic particles, such as iron and manganese ; they
generally occur on a red, brown, or black ground of chalcedony
or sapphirine. The name mocha-stone is derived from Mocha,
in Arabia, where it was found. There are other varieties,
which are known by the names of jasper-agate, -wood-agate
(wood petrified by agate), undulata or zone-agate, and brecciated agate. What are called Siena agates, seen in the Florence gallery, present a dark ground diversified with white,
gray, brown, and yellow clouds.
The best specimens of agates are brought from India, and
the second in quality are from Uruguay and Brazil ; those
found at Brighton, England, the Isle of Wight, Chamounix,
and Niagara, are really the same in kind as the Indian and
Brazilian, but differ from them by being water-worn. They
are plentiful in the United States ; one of the most noted
localities for this gem is at Agate Bay, Lake Superior, where
small, red, banded specimens are found, while large and beautiful agates have been discovered in the Rocky Mountains and
in Colorado. The collection sent from that State to the New
Orleans Exposition included many fine varieties. Of all
American gems, the moss-agates are the most abundant and
the cheapest ; those found in streams, called " river-agates,"
are considered the best. They are nearly all sent abroad for
cutting.
