Chaos Ring

The Chaos Rings are a group of powerful rings that appear in Knuckles' Chaotix. By the events of the game, Eggman has obtained most of them and is setting his plans in motion. During the game, there are six remaining Chaos Rings to collect by accessing Special Stages, and only by collecting all six will the player be rewarded with the game's good ending.

History
According to the game's Japanese manual, the Rings were created by an ancient civilization and infused with Chaos Emerald energy. Like the Chaos Emeralds, they can used for evil purposes in which they become Dark Rings.

Doctor Eggman 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 appear during the boss battles when the Chaotix fight Robotnik, but after Robotnik is defeated, the Dark Ring falls out and disintegrates. After Metal Sonic loses to the Chaotix, he appears before Robotnik and the two leave off-screen with the last Dark Ring; the game is unclear whether or not they merely escaped or if Metal Sonic transformed into Metal Sonic Kai (the final boss) with the power of the Dark Ring.

The Chaos Ring storyline was largely unmentioned in the US manual, and instead Eggman was said to be trying to steal the Power Emerald from Carnival Island. The Chaos Rings can still be collected, but are never explained. 

Trivia

 * Chaos Rings do not trigger a Super transformation once collected.
 * During the Knuckles' Chaotix special from Archie Comics, the Chaos Rings were replaced with Power Gems.
 * Chaos Rings were originally known as Holy Rings during development.
 * The Chaos Rings are similar to the Seven World Rings minus a white chaos ring.