Time Stone

In Sonic CD, the seven Time Stones were used instead of Chaos Emeralds. These mysterious gems teleports the Little Planet through time, and as such it is only visible in the planet's atmosphere for one month out of every year. The Time Stones cannot cause transformation (or if they can, have not, meaning there might be a possibility of a "Time Sonic") and have not reappeared since their debut. If Sonic collected all of the Time Stones, he could use their power to prevent Eggman from ever spreading his influence on the Little Planet, thereby ensuring a good future for the planet.

In the "SatAM" cartoon, there were two Time Stones that were guarded on a flying island which could only be accessed by defeating the guards and answering the riddle asked by the Guardian, who rather than being Knuckles the Echidna was instead an enormous, ancient owl. Sonic and Sally Acorn used these two Time Stones to transport themselves back to the day of Robotnik's takeover of Mobotropolis during the two-part episode "Blast to the Past" in an attempt to stop the takeover from ever happening. This ultimately failed, but Sally was able to save her nanny Rosie by telling her to never leave Knothole no matter what due to the fact that Sally knew that she would be captured while out in the Great Forest and roboticized by Robotnik.

In Sonic the Comic, there was mention in a Sonic's World strip of there being seven Time Stones on the Miracle Planet, but only one was ever seen in the main strip and it resembled a normal stone rather than the multi-colored gems shown in the back-up strip. Robotnik, by means unknown, had find the Time Stone and was using it to power a machine that would cyberform the whole Miracle Planet; through a bizarre time paradox, Sonic received a Time Stone from himself so he could go back in time, retrieve the Time Stone (and destroy the machine), and then go forward in time to give the Stone to himself, after which point the machine's effect on the planet suddenly vanished, as did the Stone. Its origin was never explained.