become
opaque either by alteration that it has undergone, or by the addition
of a certain quantity of oxide of iron, or of hydrate of the same
oxide. There are red jaspers, brown jaspers, and green jaspers. In
certain circumstances, as in the Egyptian pebble, the jasper presents
irregular zones, which display a structure roughly concentric"
(Du-fr^noy).
It is one of the thousand varieties of rocks known under the name of jaspers. These varieties, hard enough to cut glass, present wide bands of diverse colours, generally red and green, upon a brown ground.
The
silicious element predominates in the jaspers, but with it is
associated certain bases (alumina, oxide of iron, &c), sufficient
to render the whole fusible under the flame of the ordinary blowpipe,
which is not the case with quartz or its varieties that are very nearly
pure.
The
substances known in commerce under the name of jaspers differ so
greatly from each other that their price varies from 20c. to $12 the
pound.
ENGRAVING UPON AGATE, &C.
It
is the agate, and the varieties of which it is the type, that have in
all ages furnished to the engraver the stones best suited to his art.
