Warp Ring

From the Sonic News Network, the Sonic Wiki

A Warp Ring is a gold ring that is much larger than regular rings. They first apepared in Sonic's first game Sonic the Hedgehog and would appear near the end of an Act if Sonic has more than 50 rings. While the Bonus Plate spins when Sonic rush by it, he will have a chance to jump at the Warp Ring teleporting him into the Special Stage.

Warp Rings did not appear in Sonic the Hedgehog 2 as access to the Special Zone has been replaced with Star Posts, but both Star Posts and the Warp Rings return in both Sonic the Hedgehog 3 and Sonic & Knuckles. The Warp Rings this time are hidden around the Zones of Angel Island instead of at end of Acts. Like before these Warp Rings teleport Sonic, Tails or Knuckles (depending on who you are playing at) to the Special Stage. (Star Posts would just take you to a Bonus Stage where you can earn power ups). When the all the Chaos Emeralds become Super Emeralds, the Warp Rings change purpose to become as valuble as 50 rings once collected.

They are also seen in Sonic 3D Blast but they appear as floating orange Lifebuoy like rings which Sonic can hold on to while collected Flicky's jump in.

Warp Rings also appear in the Archie Comics continuity, in which they were created by Doctor Finitevus. The Warp Ring is essentially a portal resembling a Power Ring. It serves as instantaneous transport for anyone who uses it, and can seemingly penetrate any security measures and allow travel to any location, regardless of distance.