414 
AGATE. 
when cut and polished to attract the attention of 
the jeweller. It is subject to almost endless va- 
riety, and is esteemed in proportion to the beauty 
and elegance of its tints. The onyx, the cornelian, 
the chalcedony, and the sardonyx, are merely va- 
rieties of this stone, which entirely depend for their 
celebrity on the beauty of their colour. The names 
which they have obtained are mostly derived from 
the Greek, as if the business of the lapidary in cut- 
ting them, and the fondness of admiring their seve- 
ral beauties and figures, had been derived from that 
nation alone. 
The globular form which agates usually assume 
is said to be owing to their situation, which is 
generally in the cavities of rocks. When this is 
not the case, the stones are either distributed with- 
out order in the earth where they are found, or else 
disposed in interrupted layers or beds. They have 
likewise been observed in veins of metallic sub- 
stances as well as in minerals of the most opposite 
nature, Saussure having noticed them in some gra- 
nite, near Vienna, while Humboldt assures us that he 
has met with veins of agate in chalk. According 
to M. Brogniart, agates are chiefly found in the 
porphyry and volcanic earths, particularly in the 
cavities of volcanic tufas, and in porous lavas. In 
these situations they line the walls of the cavities 
J 
with very pretty stalactites, and are sometimes 
formed of round globules as transparent as drops of 
limpid resin. It is thus they are met with in al- 
most all the volcanic countries, but principally in 
the Ferro Islands, and in Auvergne. 
