The Ring of Acorns is a magical artifact and extremely powerful version of a Power Ring forged by Nate Morgan with the power of a Super Emerald. It subsequently became a royal artifact once it was retrieved by Sonic the Hedgehog from the Lost Temple of Shazamazon in the Great Rainforest. Its powers were eventually used up in 3236 by Amy Rose.
One Ring Among Them All
Unlike the Crown of Acorns and Sword of Acorns, the Ring of Acorns was not forged out of the Source of All. Instead the Ring was created by Nate Morgan in his travels across Mobius. After a self-imposed exile from the Kingdom of Acorn, Morgan came across the Lost Temple of Shazamazon in the Great Rainforest while seeking a new place to call home. After somehow getting past the temple's elaborate security system, he discovered one of the last remaining Super Emeralds inside the central worship chamber. Using the emerald as an energy source and his knowledge in Power Ring replication technology, Nate forged an exceptionally strong ring, naming it in honor of his former adoptive homeland and left it in the temple in place of the emerald. (StH: #65, #67)
A Ring Wrung Out
Over ten years later, the Ring would be recovered by Sonic, who was in need of a replacement fuel source to get his plane back to Mobotropolis, and placed in the Royal Reliquary after his return. Weeks later, Elias Acorn meant to use it in an effort to restore a comatose Alicia Acorn to full health due to its powerful healing energies. However, the Ring's power was exhausted when Amy Rose made a wish on it to be older, which resulted in her physical age increasing. She also absorbed some of the ring's wish-granting energy, which enabled her to conjure her Pico Pico Hammer. Following this, the Ring became powerless, and presumably was retained only for its value as an artifact. (StH: #67,#79, #80, CSE)