Obsidian, or volcanic glass, does not occupy a high position as a gem
or as an ornamental stone, but its antiquity and occasional use among
the agates and semi-precious stones will justify its mention.
This mineral is a melted lava, and consists of silex, alumina,
and a little potassa, soda, and oxide of iron. Obsidian is 6 to 7
in hardness, has a specific gravity of 2.25 to 2.8, is sometimes
transparent but mostly translucent to opaque, and is vitreous to
metallic in lustre. It is brittle and not easily attacked by acids. It
melts before the blow-pipe and takes a high polish.
Obsidian comes from volcanoes, and is found in Iceland, Teneriffe,
Lepare islands, Peru, Mexico, Sicily, and on all volcanoes. The
color is velvety-black to gray, brown, greenish-black, yellow, blue,
bottle-green, and white, seldom red, and often with black or yellow
spots or veinings.
Iceland agate lava, volcanic lava, and royal agate are all obsidian.