This
is a natural glass which is used to some extent at the present day for
ornamental purposes. In earlier times, especially among the prehistoric
peoples of the western hemisphere, its use was very extensive, both
for utensils and ornamental articles.
Obsidian
is a product of volcanic outflows, being produced where a rapid cooling
of certain liquid lavas has taken place. In color it may be black,
gray, green, red, brown, or yellow, and in diaphaneity may vary from
transparent to opaque. The kind used almost exclusively in the arts is
of black color, generally transparent only in thin pieces.
The
properties of obsidian differ little from those of manufactured glass.
Its hardness is 5-5-1/2; specific gravity 2.3-2.5. It has a vitreous
luster, and is brittle, breaking with a large conchoidal fracture which
is quite noticeable. Its extreme brittleness makes cutting of it
difficult. It fuses rather easily before the blowpipe to a porous, gray
mass. Being amorphous it cannot be distinguished optically from glass,
like which it is singly refracting. It frequently contains partially
crystallized inclusions, however, and gas pores, which are not common
to artificial glass. When these are arranged in regular order the
obsidian shows a chatoyancy, or schillerization, which gives a pleasing
effect. In chemical composition obsidian shows a higher percentage of
alumina and a lower one of alkalies than artificial glass. The
following is the composition of an obsidian from the Lipari Islands:
Silica, 74.05, alumina, 12.97, iron oxide, 2.73, lime, 0.12, magnesia,
0.28, potash, 5.11, soda, 3.88, loss on ignition, 0.22. Obsidian is not
easily attacked by acids. One of the largest known deposits of obsidian
in this country occurs in the Yellowstone Park, Wyoming. The locality
is known as Obsidian Cliff, and the deposit has a thickness, according
to Professor Iddings, of 75 to 100 feet. There are evidences that the
Indians obtained obsidian here for use in their arts, as flaked
fragments are found in the vicinity. The color of this obsidian is for
the most part black, but shades of red and yellow occur.
A
variety of obsidian, showing red and black in alternate streaks, or
spots, occurs here as well as in other localities. This is known as
180
