Obsidian n. : A kind of glass produced by volcanoes. It is usually of a black color, and opaque, except in thin splinters.

Example:
What is the difference between the Obsidian weapons and the regular weapons in Dead Space?

Taxonomy
Obsidian, obsidian blade, volcanic glass, snowflake obsidian, free encyclopedia, lava flows, glass, rainbow obsidian, blades, obsidian tools, SiO2, dark green, lava, crystal growth, chemical composition, Origin, obsidian flow, Obsidian Dome.

Defination from the web:
obsidian blade edges can reach almost molecular thinness, leading to its ancient use as projectile ... This breakdown of obsidian is accelerated by the presence of water. ...