|This article is about an older version of DF.|
Materials have a sharpness property, defined as [MAX_EDGE:<number>]. In combat, this presumably defines how well the material works when used for slashing attacks.
Most materials draw this from their Material Template, but placing a [MAX_EDGE] tag in the material's entry will override this and allow it to be sharper than other objects of its kind. For example, Obsidian has [MAX_EDGE:20000], while the Stone Material Template has [MAX_EDGE:1000]. Incidentally, it seems that this is what allows Obsidian to be made into stone swords; a value of at least 10000 has been proven to also work.