Obsidian from the Rocce Rosse lava flow on Lipari island (Italy)
Detail of obsidian with spherolites (small bubbles where the glass has crystallized around the bubble)
Black volcanic glass
Obsidian is a naturally occurring glass formed as an extrusive igneous rock. It is produced when felsic lava extruded from a volcano cools rapidly through the glass transition temperature and freezes without sufficient time for crystal growth. Obsidian is commonly found within the margins of rhyolitic lava flows known as obsidian flows, where cooling of the lava is rapid. Because of the lack of cr... -> See whole entry
