Red Star Ring

Red Star Rings (レッドスターリング), also known as Special Rings (スペシャルリング), are unique Rings found in several games in the Sonic the Hedgehog series. Red Star Rings are often used to unlock special bonuses or to allow entry into the Special Stage.

Sonic Advance 2
Special Rings were first introduced in Sonic Advance 2, in this game, Special Rings are slightly larger and more yellow than regular rings with a star in the middle and wings on each end on the hoop. There are seven Special Rings in each act and all seven must be collected to allow the player to enter the Special Stage. Special Rings are not kept after each playthrough of an act, meaning that if the player fails to collect all seven Special Rings, they must collect all seven the next time they play through the act.

Sonic Colors
The Special Rings appear once again in Sonic Colors as collectable items. In this game, they are colored red and are missing the wings they had in Sonic Advance 2. They are found in both the Wii and Nintendo DS versions. Five Special Rings are found in each level in both versions. Special Rings will also give a Special Bonus of 30,000 points. In the DS version, if all are collected in a stage or world, pictures in the gallery are unlocked. When all of the Red Star Rings are collected in the DS version, Sonic unlocks the Infinite Boost. When all the Red Star Rings are collected in the Wii version, all the stages in the Sonic Simulator are unlocked. When all of the Sonic Simulator stages are completed, Super Sonic mode is unlocked.

Sonic Generations
The Special Rings return in Sonic Generations, where they are now known as Red Star Rings. There are 5 Red Star Rings in each act, collecting one unlocks music or concept art. If all the Red Star Rings are collected in an act, one giant Red Star Ring will hover over the stage and a new skill will be unlocked.

If you talk to a friend when they have the icon of the Red Star Ring above them, they will give you a hint as to where they are. Red Rings are not present in the Nintendo 3DS version.

Sonic the Hedgehog 4: Episode II
Red Star Rings make an appearance in Sonic the Hedgehog 4: Episode II, the only purpose they serve in this game is for 100% completion as well as an achievement/trophy.

Sonic Jump (2012)
In the 2012 remake of Sonic Jump, there are three Red Rings to collect in each act.

Sonic Dash
Red Star Rings appear rarely along the courses in Sonic Dash, but they can also still be obtained by completing certain objectives and can be used to unlock new characters or to buy retries for whenever you fail.

Sonic Lost World
Red Rings reappear in Sonic Lost World and are used to unlock content. Some Red Star Rings can be obtained only when Sonic uses the Parkour system and some of them come out of bells Sonic chases along from the stage. Collecting all the Red Rings in the Wii U version unlocks the Zone's Chaos Emerald. However on the 3DS version, collecting  all the Red Rings in a World will unlock a Bonus Zone.

Trivia

 * In Sonic Advance 2, Special Rings can be collected by using Cream the Rabbit's Chao Attack.
 * According to Blaze in Sonic Generations, there are objects like Red Star Rings in her dimension.