Chaos Emeralds
From Sonic Retro
Seven (or six in Sonic 1) mystical emeralds that allow Sonic to transform into Super Sonic when used in conjunction with fifty Rings.
They are also used as plot points in several games.
Chaos Emeralds are earned in Special Stages, varying between games both on type and method of getting them.
Super Emeralds, which have as yet only appeared in Sonic the Hedgehog 3 & Knuckles, will enable Sonic to transform into Hyper Sonic.