Obsidian (3.5e Equipment)
From D&D Wiki
Obsidian is a type of naturally-occurring glass formed as an extrusive igneous rock. It is produced when felsic lava erupted 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. Eyewitness accounts from Spanish Conquistadors shows that some obsidian blades were capable of cleanly decapitating a horse in a single blow.
Obsidian is extraordinarily sharp, and can do grievous damage. Against creatures who have a Constitution score, a masterwork obsidian weapon in the hands of a proficient wielder does additional lethal damage equal to 1/2 of the weapon's dice.
Spears made from obsidian have obsidian heads and wooden shafts.
The disadvantage of obsidian is that it is more brittle than steel, and therefore more prone to being sundered. Obsidian has 15 hit points per inch of thickness and hardness 10.
Some of the descriptive text has been drawn from the wikipedia article on obsidian.