Emerald Staff

Emerald is common enough that this is often the gem staff you can build rather than the one you would pick. The bolt goes through one enemy before it dies, so it is at its best in a corridor where things queue up behind each other and worst in an open cavern.

It will carry you through the early caves and then fall off hard the moment Hardmode ore or Dungeon loot appears. The mana cost per shot is the real constraint: without a potion supply or a star arena to stand in, you will run dry partway through anything that fights back.