Obsidian Chunk

Credit: Rob Lavinsky, iRocks.com – CC-BY-SA-3.0 [CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons

This image is originally part of the article Natural Glass, 4 Stunning Anomalies

A raw chunk of obsidian with cristobalite properties. Made when lava high in silica cools rapidly.