An Etherium Sword is a weapon added with the SteamCraft mod. It is the 3rd strongest sword, and is exactly the same strength as the Diamond Sword. Since it is made of Etherium Crystal, it does not decay. Since Glowstone and certain other blocks are broken quicker with a sword, it would make sense to use the Etherium Sword to break these blocks since it is immune to the x2 damage. Care must be taken in the Nether, however, as it is still vulnerable to Lava when dropped.