Chaos Emeralds (Sonic Underground)

The Chaos Emeralds are objects that appear in Sonic Underground. It is a rare and powerful green emerald variety of unknown origin that is found on Mobius. So far, only two such emeralds have been discovered.

Past
The first known Chaos Emerald on Mobius ended up on the Floating Island where its power is used to keep the island aloft in the sky. In the present day, it is protected by its guardian Knuckles the Echidna. Most of Mobius believed though that this Chaos Emerald was the only one in existence.

Friend or Foe?
Seeking to steal this Chaos Emerald for Dr. Robotnik, Sleet and Dingo tricked Knuckles into going after the Sonic Underground while they stole the Chaos Emerald. Before they could get away with the Emerald though, Knuckles and the Sonic Underground got it back, preventing the Floating Island from crashing onto Mobius.

Chaos Emerald Crisis
Dr. Robotnik eventually discovered a second Chaos Emerald which he used to power his Fortress of Altitude. When the Sonic Underground tried to destroy the fortress, the Emerald's power was used to keep the trio away. Fearing roboticization since the Fortress of Altitude made their services unrequired, Sleet and Dingo stole the Chaos Emerald to seize power. However, as the duo fought over who should rule Mobius, the Chaos Emerald was broken, releasing its destructive energies on Mobius.

In an attempt to secure the Chaos Emerald fragments, Sleet tried to contain them inside Dingo - transformed into a safe. However, the power proved too much. Reacting with Dingo's mutation abilities, Dingo turned into a giant, shape-shifting monster. With Dingo now threatening to destroy Mobius, the heroes and Dr. Robotnik worked together to make Dingo spit out the Emerald fragments which Knuckles then contained in a canister from Althair, thus restoring Mobius to normal. Afterwards, Knuckles brought the Emerald fragments to the Floating Island.

Powers and traits
The Chaos Emeralds possess mystical properties and contain seemingly unlimited amounts of Chaos energy, an unstable and highly volatile form of green energy, making them potent energy sources. Even a single Chaos Emerald holds enough power to destroy Mobius. If a Chaos Emerald is broken, its leaking energy can be apocalyptic to Mobius, causing wild energy storms that can be seen from space and seismic earthquakes.

When controlled, a Chaos Emerald's energy can be harnessed to power machinery or keep landmasses afloat. Its power can also be used to empower beings, although a person will mutate and gone insane as a side-effect.

Trivia

 * Dr. Robotnik's plot to steal the Chaos Emerald from the Floating Island occurred in similar way to the Archie Comics story of the same basic idea.
 * Despite being the Chaos Emeralds of the Sonic Underground series, they bare apperances and powers more similar to the Master Emerald. Both emeralds are green and fairly large and their primary use is to keep large landmasses airborne.