An argonite bar is a bar of refined Argonite ore. It can be created through the Smithing skill by using argonite ore on a furnace, requiring level 60 Smithing and granting 44.8 Smithing experience. The argonite ore itself can be mined from the rocks scattered throughout the dungeon, obtained as a monster drop, or bought from the Smuggler.
An argonite bar can be smithed on an anvil through the Smithing skill to create Argonite equipment, granting between 64 and 68.5 Smithing experience per bar used. A list of argonite items that can be smithed from argonite bars and the Smithing levels required can be found here.