Obsidian is a natural volcanic glass that forms when lava solidifies so rapidly that mineral crystals do not have time to grow. Its color is usually black, but sometimes with the presence of hematite (iron oxide) it can be Indian red to brown. Obsidian has a bright, vitreous luster on smooth surfaces, and is sometimes bonded, with spherulites. Obsidian has the same chemical composition as granite, syenite, or granodiorite.
Obsidian is found in the outer edges of rhyolite domes and flows. Like rhyolite, obsidian can show flow-banding. It is a fairly soft gemstone and can be easily scratched and cracked. The color of obsidian can be uniform or patterned, and can have twisted or straight color bands caused by the solidification of flowing lava. Pitchstone obsidian has a resinous luster that forms as the rock absorbs water over time. Snowflake obsidian is well known for the white marks that resemble snowflakes, which are caused by internal bubbles or crystallites.
Obsidian was used by American Indians and many other ancient civilizations to create ornaments, cutting tools, arrowheads and spearheads. The Aztecs and Greeks used it for mirrors because it is so reflective.
Obsidian is found worldwide, but particularly in Scotland, the Lipari Islands off the north coast of Sicily, Italy, Hungary, Russia, Iceland, Japan, New Zealand, Mexico, and in the United States in (Yellowstone Park, California, Oregon, Utah, New Mexico and Hawaii).