The Chaos Emeralds

Throughout history, the Chaos Emeralds have been the center of countless world-threatening conflicts and have been targeted by multiple factions on both earth and beyond, who seek them for their near-limitless powers. In the hands of villains they have been used for doomsday weapons and schemes for world domination, and in the hands of heroes they have been used to save the world. Over time, the Emeralds have been gathered multiple times, though they tend to scatter themselves after each usage and reset the hunt for them.[4]

In the early games of the series, the Chaos Emeralds were more like "bonus" items, but since Sonic Adventure, they have become necessary plot devices, and that trend has continued throughout the later games.