Obsidian has the same chemical composition as granite, but it is quenched to make glass rather than cooled slowly to form crystals.
Most obsidian is black, or black with red bands.
It is brittle. Indians used it to make arrowheads and knives. Marilyn M. Lindstrom, Ph.D., Planetary Scientist, National Aeronautics and Space Administration, Johnson Space Center. How to cite this article.