From Sonic Retro
The Time Stones (タイムストーン) are Sonic the Hedgehog CD's equivalent of Sonic series mainstay the Chaos Emeralds. The seven Time Stones are found on Little Planet, and allow anyone who can collect them all to gain control of time itself. In the game, Dr. Robotnik tries to collect them in order to take over Little Planet, and Sonic the Hedgehog tries to collect them in order to save the Little Planet from him.
The Time Stones are collected during gameplay by Sonic entering Special Stages (via Giant Rings, which are found behind the Robotnik Signpost if he is holding at least 50 normal Rings) and finding them therein. If the player is able to collect all of the stones early in the game, there will be no need to travel to the past, because the future automatically becomes good (i.e. free of Robotnik's Badniks and other mechanisation. If Sonic does not have all the Time Stones, he must travel to the past version of each Round's Zones 1 and 2, and undo the damage that Robotnik did in the past. Should the player opt to do this, it is possible to obtain the good ending without collecting a single Time Stone.
In the Archie comics, manipulation of the Time Stones is referred to as Chronos Control (StH #215); the temporal analogue of Chaos Control. This power is used by Silver the Hedgehog in the comics, thus making the time travel of his story more logical than it was in Sonic '06 (in which two entities simultaneously using Chaos Control with at least one Chaos Emerald each could create a time portal).
The Time Stones shown on the Special Stage results screen are markedly different from those shown in the Special Stages when Sonic obtains them. On the former, they have 8 sides and resemble the Chaos Emeralds of Sonic the Hedgehog; in the latter, they have 6 sides and resemble the Emeralds of Sonic the Hedgehog 2.