Obsidian

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 crystal structure, obsidian blade edges can reach almost molecular thinness, leading to its ancient use as projectile points, and its modern use as surgical scalpel blades.
Obsidian represents the Ages of Abandonment,Halo: Contact Harvest, page 196 a period of time that begins the Covenant's Progression of Ages.
==Sources==
Category:Materials
Category:Geology