Time Stone

The Time Stones are a group of seven stones that can be used to travel through time. They replaced the Chaos Emeralds as collectible objects in Sonic CD. These mysterious gems teleport 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) and have not reappeared since their debut. Their similarity to the Chaos Emeralds is striking, in appearance as well as their ability to manipulate time and space, the only major change being that they are much larger. However, the major difference between the Time Stones and Chaos Emeralds is that the Time Stones only seem to be able to warp through time on Little Planet itself. It is implied that they cannot leave Little Planet, or that they have no outside influence. 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.

SatAM
In the "SatAM" cartoon, there were two Time Stones that were guarded on a floating 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 as Sonic didn't keep his mind on the exact date, but Sally was able to download the original roboticizer plans from one of Robotnik's terminals and saved 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).

Sonic the Comic
Time Stones also appeared in Sonic the Comic in the UK. They were first mentioned in a Sonic's World back-up strip, in which it was revealed that there were six Time Stones rather than seven (at the time, the series also included only six Chaos Emeralds). The strip also mentioned that they were hidden somewhere on the Miracle Planet and that Doctor Robotnik wanted to find them so that he could control time and become unstoppable.

One such Time Stone appeared as part of a bizarre time paradox during the series adaptation of Sonic CD. Whereas the Time Stones that had appeared in the back-up strip appeared to be multi-coloured gemstones (as in Sonic CD), the one in the main story looked like a normal, oval-shaped grey stone, although it glowed when it was being used.

Robotnik had somehow located a Time Stone, and used it to power a machine that cyberformed the entire Miracle Planet in the space of a single month (between the planet's appearances over Mobius). Later, after Sonic had chased Metallix to the Miracle Planet in order to rescue Amy Rose, Sonic received the Time Stone from a duplicate version of himself from the future. He used it to travel back to a time before the planet had been cyberformed (although the Time Stone disappeared when he did so). He retrieved a past version of the Time Stone from Robotnik's machine (destroying the machine in the process), then travelled forward in time again to give the Stone to his past self (who then travelled back in time, as part of a closed loop). After Sonic's past version had left, the machine's effect on the Miracle Planet suddenly disappeared, as did Metallix.

Neither the Stone's origin, nor its eventual fate after its disappearance, were ever explained.

Sonic Universe
In Sonic Universe, Ark 2, 30 Years Later, Silver the Hedgehog enters and leaves King Sonic's time using a yellow gem similar to a Time Stone. Silver uses the it to perform Chronos Control, a similar ability to Chaos Control that is limited to affecting time.