Emerald swords are the most useful weapons in Minecraft. Ender Steve holds emerald swords in the End. It is the hardest sword to obtain, as it requires two emeralds to craft, which are very hard to find.
A single emerald sword deals two hearts more damage than a diamond sword and has twice the durability (3124 hits). It can kill most mobs in one hit. When mining dirt, sand, and other soft materials with an emerald sword, mining goes 1.5x faster than when mining with one's hands. The only mob that spawns holding this sword is Ender Steve.