Some special materials for weapons and ammunition are described in Armory page 88. Additinoal materials are described here.
Some Native American legends tell of the awesome cutting power of weapons fashioned from shards of obsidian. This glass-like stone is, indeed, very sharp and can make a mean blade. Knives and swords made of obsidian gain a +1 bonus when the target has no armor, and obsidian scalpels gain the 9-again benefit on surgery rolls involving fine incisions.
The extremely fine edge of obsidian comes at the expense of durability. Obsidian blades have the Fragile 2 quality: when used in combat, any attack that deals more than 2 points of damage shatters the glassy blade, which usually ruins the weapon.
Obsidian arrow heads are covered on Armory page 90 and obsidian bullets function like soft stone ammunition, described on Armory page 89.