The Chaos Emeralds are seven emeralds (six in the original, eight in Sonic The Fighters, numerous emeralds in Sonic Spinball, and seven in every other game) with mystical powers, and a recurring feature of Sonic games. According to official sources, the emeralds can turn thoughts into power,[39] give energy to all living things, and be used to create nuclear or laser based weaponry.[40]
They are the basis of many of the games' plots, and the player is frequently required to collect them all to fully defeat Doctor Robotnik and achieve the games' "good endings", super forms, or both. The method used to acquire the Emeralds and the result in collecting them differs between titles in the series. Most early games require the player to find the emeralds in Special Stages.[38] In some games, such as Sonic R and the 8-bit versions of Sonic the Hedgehog and Sonic the Hedgehog 2, the emeralds can be found in hidden locations within the main levels. In most later games, the Chaos Emeralds are acquired by the characters during the course of cut scenes in the games' story modes, and do not need to be collected by the player. The Chaos Emeralds have gone through various redesigns as the series evolved, beginning as hexagons and later being portrayed in the brilliant cut shape.
In Sonic the Hedgehog 2, [[Sonic the Hedgehog 3, Sonic & Knuckles and Sonic 3 & Knuckles, gathering all the Chaos Emeralds allows Sonic to turn into Super Sonic by collecting 50 rings and jumping in midair. Its effects are similar to a combination of the Invincibility and Super Speed power-ups. Whilst transformed, the ring count slowly drains, and when it reaches 0, Sonic reverts to his normal form. In Sonic & Knuckles, Sonic 3 & Knuckles and Sonic the Hedgehog 2 featuring Knuckles the Echidna Knuckles can also transform into Super Knuckles. In Sonic 3 & Knuckles, players can obtain Super Emeralds that enable the player to transform into Hyper Sonic, Hyper Knuckles and Super Tails forms, which take on character-specific abilities in addition to the properties of Super Sonic.
In Sonic Adventure 2, Shadow the Hedgehog showed that the Chaos Emeralds have the power to control time and space (Chaos Control) and striking foes (Chaos Spear). Also in Shadow the Hedgehog (video game) he shows that the Chaos Emeralds can destroy anything and everything within a 20 yard radius (Chaos Blast). In Sonic the Hedgehog (2006 video game) the Chaos Emeralds show the power to travel through time and to shoot bullet like energy beams (Chaos Lance).
There are various versions of the Chaos Emeralds in different media. In the back-story for Sonic's world featured in most European tie-in publications such as Sonic the Comic, the Chaos Emeralds have the property of absorbing "evil energy", and play a role in the origin of Dr Robotnik. In Sonic Rush, the Sol Emeralds from Blaze the Cat's alternate dimension perform a similar function to the Chaos Emeralds of other games. In Adventures of Sonic the Hedgehog, there are four Chaos Emeralds of varying shapes, representing invisibility, invincibility, immortality and the power of life. Chaos Emeralds also appear in Sonic Underground, with an appearance similar to the Master Emerald shards of Sonic Adventure.
Each Chaos Emeralds has a distinct colour, although the exact hues used vary between the games; usually the seven emeralds are red, blue, yellow, green, white, sky blue and purple. The Sol emeralds from Sonic Rush also come in these colors.