The Maerum's piece of the Stone Disk. It is kept in a secret niche inside the Maerum's abandoned temple, and is obtained when April puts the Talisman of the Balance against it. Actually, it is only half of a piece: it is only complete when put together with the Alatien Stone. The shape of this half is reminiscent of a Maerum's fins.
When reassembled, the Maerum-Alatien stone looks like two hands clasping a core, symbolising how the two races are reliant upon, and completed by, each other.