The Chaos Emeralds are items of great magical power in
Sega's
Sonic the Hedgehog universe. In just about every
Sonic game
Dr. Robotnik/
Dr. Eggman steals the Chaos Emeralds to power his latest and greatest scheme to take over the world. It's
Sonic's job to recover them and stop the
evil doctor. Collecting them all results in the happy ending when you finish the game. Failure to collect them all usually results in a screen saying "Try Again". The Chaos Emeralds are typically found in secret bonus rounds, but in
Sonic the Hedgehog for the
Sega Master System and
Game Gear the emeralds were just sitting out in the open. In every other instance
Sonic has to nagivate a maze or half-pipe of some kind, collecting rings and dodging spikes, until he finds the emerald somewhere in the region.
Starting in Sonic the Hedgehog 2 for the Sega Genesis (and later for the Nintendo GameCube in Sonic Mega Collection) and continuing on to Sonic the Hedgehog 3, Sonic and Knuckles, Sonic 3 and Knuckles, and Knuckles the Echidna in Sonic the Hedgehog 2 something wonderous happens when Sonic collects all six emeralds. If he collects 50 rings and jumps into the air, he becomes Super Sonic and gains speed and invincibility. However the ring count counts down, and when it hits zero Sonic reverts to normal. It should be noted that both Tails and Knuckles the Echidna can use the emeralds to gain special powers, becoming Super Tails and Super Knuckles, respectively. If you combine Sonic the Hedgehog 3 and Sonic and Knuckles and gain all the Chaos Emeralds and Master Emeralds, the characters can activate the hyper powers (resulting in even more speed), becoming Hyper Sonic, Hyper Tails, and Hyper Knuckles.
When Sonic made the leap to 3D in Sonic Adventure for the Sega Dreamcast he battled against Dr. Eggman once more over the fate of the Chaos Emeralds. This time around the mad doctor was feeding the emeralds to an elemental being of pure chaos. This creature (named, of course, Chaos), actually drew power from each emerald that it injested. Sonic and friends did their best to retrieve the emeralds before Eggman could, but in the end a super-powered Sonic had to fight a fully powered Chaos over the fate of Station Square and beyond. This story was retold in the Nintendo GameCube port of the game, Sonic Adventure DX. The Chaos Emeralds were only indirectly featured in the game's sequel, Sonic Adventure 2 (and the Nintendo GameCube port Sonic Adventure 2 Battle), as the evil Shadow the Hedgehog makes us of an emerald to activate the Chaos Control, his secret method of warping time and space for his own benefit.
Sonic and friends went back to their gaming roots in Sonic Heroes for the Nintendo GameCube, Sony Playstation 2, and Microsoft Xbox. In this latest 3D Sonic game a total of seven Chaos Emeralds must be collected in order to unlock the mysterious Last Story level. The emeralds can be earned by completing a race across a full pipe level (similar to the half pipe from Sonic the Hedgehog 2), and the race can only be accessed if a team picks up one of the special keys hidden in every other level. An unrelated eighth emerald makes a cameo appearance when Team Dark (consisting of Shadown, Rouge the Bat, and Omega-123) activates the Chaos Control special attack.
Paralleling the plot of Sonic Adventure, the Game Boy Advance title Sonic Battle features a mysterious robot by the name of Emerl who is powered by Chaos Emeralds. Sonic, his friends, and his enemies are all racing to capture the emeralds to power Emerl up to his full potential. In the end, however, Emerl reaches full power with the aid of all seven emeralds and turns out to be a greater force than anyone possibly imagined.
It should be noted that in the 32X release Knuckles Chaotix Knuckles and friends were searching for the Chaos Rings, an item that may or may not be related to the Chaos Emeralds. Sonic CD for the Sega CD and PC featured Sonic tracking down the Time Stones which appear to be unrelated to the Chaos Emeralds.