Chaos Emeralds are powerful gems in the Sonic series. There are 7 Emeralds and if someone collects all 7 Emeralds they will become unstoppable! The Chaos Emeralds can also be used power machinery, e.g. the Death Egg, Metal Sonic and the Tornado, with enough power, you could turn them into Super Emeralds, godlike gems which can transcend you to a Hyper form.
Sonic Speedball series
Chaos Emeraldss were planned to be in the entire Nightwolf series, but only appeared in Nightwolf the Game. By then, Omegaverse Corp. thought to include them in one more game, Hyper Nightwolf, but it didn't happen.
They appear in the Micool series that can make Mick Cool transform into Super Mick Cool and can Micool super as well. But if Micool is enraged or depressed when using the emeralds he will transform into Dark Micool.
Chaos Emeralds make an appearance in Sonic Heroes 2 as power-ups for Chaos Emerl. They are present in Aqua Lake Zone, Great Turquoise Zone, Sandopolis, Casinopolis, Scrap Brain Zone, Station Square and Sleeping Egg Zone.