Obsidian

Obsidian, igneous rock, also called volcanic glass. It is formed from molten lava. Obsidian is composed of the same chemicals as granite, but it solidifies so quickly that there is no time for crystals to form, so it becomes smooth and glasslike. Obsidian is generally black in color, but it can be brown or red. It can be shattered into sharp-edged pieces. The Native Americans of the western United States used such fragments for arrowheads and spear points.