I've been studying the raws for various metals and glass, and I'm getting some conflicting results. Glass, with its high maximum sharpness, low density, and various other features, is in theory far more deadly as an edged weapon than steel, except for its low torsion and shear strength. Given the torsion and shear strength differences, I'm now wondering how a glass serrated disc or giant ax compares to one made of steel, iron, bronze / bismuth bronze, and copper.
Can anyone help me with this?
The most important properties for edged weapons in DF are the [SHEAR_*] properties. The code compares the weapon material to the material being cut (armor, flesh, bone, etc.) based on the SHEAR_YIELD and SHEAR_FRACTURE of both materials. The damage done afterwards does depend on sharpness, weight, etc, but if the weapon material doesn't have a higher shear value then it acts as a blunt weapon rather than a edged weapon.
According to wiki, glass has [SHEAR_YIELD:33000] [SHEAR_FRACTURE:33000]. Edged glass trap weapons in DF will easily cut through cloth, leather, and flesh, but have a hard time against bone or any metal armor. Glass is a better weapon material than wood though. A low quality copper disc is better than any glass disc.
Likewise, the reason the default obsidian short swords can't cut people is that they have the default stone values of [SHEAR_YIELD:15000][SHEAR_FRACTURE:15000]. Modding shear increases for obsidian to those of glass will make the obsidian swords work properly. Flesh has [SHEAR_YIELD:20000].
Torsion values are not used, or are perhaps only used in wrestling. I don't think you'll see a creature with glass bones anytime soon.