Rainbow obsidian

Rainbow obsidian was an obsidian variety in which all colors except yellow were included in the black or gray base, sometimes in pronounced bands.

Description
These semi-precious stones may or may not have had spangles, but were usually tumbled into irregular gemstones in either case. Like other obsidian, the rainbow variety was hard but brittle, and rarely found use in places that would receive some wear. A typical specimen had a base value of 50 gp.