Chaos Ring

The Chaos Rings are a group of powerful rings infused with the power of the Chaos Emeralds. They serve as the main focal point of Knuckles' Chaotix. The Chaos Rings have very much in common with the Chaos Emeralds, they must be obtained through Special Stages and they also seem to have some kind of great power behind them. But just like the Chaos Emeralds, they can used for evil purposes in which they become Dark Rings. Dr. Robotnik discovered the rings once Newtronic High Zone appeared out of the ocean. He established a base to find as many Chaos Rings as he can. The Dark Rings also appear during the boss battles when the Chaotix fight Robotnik, but after Robotnik is defeated, the Dark Ring falls out and disentergrates. After Metal Sonic loses to the Chaotix, he appears before Robotnik and eneters the newest Dark Ring, with the power of the Dark Ring, Metal transforms into Metal Sonic Kai, who serves as the final boss of the game. It is currently unknown what happened to the Chaos Rings after the events of Knuckles' Chaotix.

Trivia
Unlike the Chaos Emeralds, the Chaos Rings can't trigger a Super transformation.

During the Knuckles' Chaotix (Archie comic) special, the Chaos Rings were replaced with Power Gems.