The Coin Ring is a Hardmodeaccessory created by other accessories exclusively dropped from pirate enemies during Pirate Invasion. It causes enemies to occasionally drop 1 to 10 more coins when hit and increases the pickup range for coins - the two effects provided by the materials used to create it. This can be useful for picking up money the player would otherwise miss and making farms more efficient.
This accessory is probably the single most useful money-making item in the game. In particular, it is possible to wait in safety while minions defeat enemies just outside your shelter, and the coins dropped go directly into your inventory. Using a swarm of particularly weak minions (like a BrokenSlime Staff) against an event with no casters (The Frost Legion is particularly safe) can rake in multiple platinum coins.
You can also farm in a Solar Eclipse, but the Mothrons and Reapers can travel through walls to hit you, so it is recommended to farm before defeating Plantera.