Two Anvils in the world are available to use before the player has crafted their own. They are located in front of the Blacksmith next to Beltran, and in the Miner's Camp on top of the Town Cave between John Stone and Steven.
The Anvil recipe will not be available until completing the quest "A New Anvil".
To use an Anvil, walk up to it with a Geode or Fallen Star in the active hotbar slot, and press E.
The Anvil can only break open 1 item at a time. To open geodes or stars in bulk, the Geode Processor is required.