- You may be looking for Air Necklace, a device with a similar function.
An Air Charm is a mystical amulet that allows air-breathing Mobians to breath underwater. Air Charms were first known to have existed in the city of Meropis, being kept at the Eusebes Shrine. While Air Charms are valuable, their abilities are not unlimited, and they must be mystically recharged in order to continue providing air. (StH: #260, #261, #262)