cent
obsidian ; there is rarely more than one color in the same specimen
with stripes and specks. Obsidian scratches white glass indifferently,
but is scratched by topaz; its streak-powder is white; it has a
specific gravity of 2.34 to 2.39. Obsidian is sometimes magnetic, so
that small pieces show their magnetic poles. Before the blowpipe, the
black variety is fusible with much difficulty; and even at a white heat
it does not melt into a solid glass; but the gray and brown variety
(marekanite) swells readily into a spongy mass.
Obsidian consists of silex, alumina, with a little potassa, soda, and oxide of iron.
The
names, Iceland agate, lava, black-glass lava, volcanic lava, are all
synonymous, and the mineral called bottle-stone, in round grains of the
size of a pea, is nothing but a green obsidian.
Obsidian,
sometimes, forms the cement of .whole mountain chains, often forms
deposits in the trachyte and the streams at the foot of some volcano;
also, among the volcanic ejections, and occurs in loose lumps in the
sand of rivers, and at the foot of mountains. It is found in Iceland,
Tenerife, the Lipari Islands, Peru, Mexico, Sicily, Hungary, Asiatic
Russia, the Ascension Islands, and on all the volcanoes of former and
present times.
In
the New York Lyceum of Natural History are several interesting
specimens, presented by Don Correa, of Tabasco, from the ruins of the
city of Palenque; such as concave or triangular wedges, and other
masses of obsidian, from various localities.
It
is employed for several useful and ornamental purposes ; such as the
making of ear-rings, necklaces, brooches, snuff-boxes, knife handles,
&c. It is particularly worn as mourning jewelry; it requires,
however, much care in working, being extremely brittle. It is ground on
lead
