The Chaos Emeralds are a group of seven emeralds, they grant the user the ability to transform into a Super version of themselves. The emeralds have an endless power source, and are able to float in the air. They're usually found in Special Stages. They can emit both positive and negative energy so some could use one side more than others. In Sonic X, Sonic transformed into a dark, Super form so that he could save Chris Thorndyke and Cream the Rabbit.
 Games Background
The Chaos Emeralds were introduced in the original Sonic The Hedgehog, however, there were only six and they had no effect to Sonic. They simply disappeared at the end of the game. In Sonic The Hedgehog 2, they became seven emeralds and Sonic gained the Super form once he had collected them. They appeared in Sonic The Hedgehog 3, but the only change was in their color and shape.
In Sonic and Knuckles, there was a set of 14 emeralds that had the ability to change the seven Chaos Emeralds to give Sonic the Hyper Sonic (an upgrade from Super Sonic) form.
Since Sonic The Hedgehog 2, the Chaos Emeralds have been used in many games, for many different many reasons.
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