From Sonic Retro
The Chaos Emeralds are a collection of mystical gemstones that appear as major plot elements throughout the Sonic games (and in most other continuities too). Even individually, these strange jewels can channel miraculous powers unmatched by almost anything in the universe; enabling their possessor to manipulate time and space, as well as unleash incredible destructive forces. Collect all seven and a miracle's supposed to happen.
The Emeralds' awesome abilities inevitably mean that they are coveted by every supervillain on Sonic's world (and beyond), so a great many of Sonic's adventures involve keeping them out of the pudgy hands of Doctor Robotnik. Even discounting their supernatural powers, the Chaos Emeralds' glittering beauty (and cash equivalent value) makes them top targets for the more financially-motivated characters as well; namely Nack and Rouge.
Chronologically, the Chaos Emerald's first appearance in the series was at the Mystic Ruins emerald shrine, during the time of Tikal and Pachacamac; this is also where the Emeralds earned their "chaos" name, on account of the destruction and mayhem they brought to the Knuckles Clan. In modern times, the Emeralds can be found in a variety of different places, but usually located in a Special Stage - bizarre pocket dimensions that the gems seem to weave around themselves.
The number, color, and shape of the Emeralds were at first inconsistent between games, but from Sonic 3 onward, they were 'finalized' as being 7 brilliant cut gemstones, colored red, green, blue, cyan, purple, yellow, and grey.
Sonic 1Sonic the Hedgehog for the Sega Mega Drive, 6 Chaos Emeralds were located in the 2D rotating Secret Zone. The US Manual states:
Obtaining all 6 Emeralds was necessary to obtain the "Good Ending". Their purpose was not explained during the game, as Super transformation was not implemented in this game.
Sonic 2Sonic 2 for the Mega Drive was the first game to feature 7 Chaos Emeralds. This number has been used by the majority of the games since. They appear different, however: They now have 6, rather than 8, sides (from the 2D viewpoint of the player). The 7th Emerald was given the color purple. The others are the same color as the ones in Sonic 1 (see above), with the exception of the blue Emerald, which is now a lighter color.
In this game, the Emeralds are central to the plot: Dr. Robotnik has taken control of much of the island, and Sonic must obtain them before he does. Sonic and/or Tails must go through a pseudo-3D half-pipe Special Stage to get them this time.
This time, getting all 7 Emeralds not only unlocks the "Good Ending", but it also enables Sonic to transform into Super Sonic, who has been featured in many games, and virtually all other Sonic-related media since then. Tails is unable to transform.
Sonic 3 & Knuckles
Sonic and Tails, with the 7 Emeralds from the end of Sonic 2, come to Angel Island when they see Robotnik has crash landed there. Robotnik tricks resident echidna Knuckles into stealing the Emeralds from Super Sonic. Knuckles steals and then hides the Emeralds in 7 pseudo-3D Special Stages, which rotate when you turn. You must collect all the Blue Spheres located in the stage to earn the Emerald.
Upon obtaining all 7, Sonic may transform into Super Sonic, as in Sonic 2, while Tails again cannot transform. Knuckles can also transform into Super Knuckles upon obtaining all 7 Emeralds.
Sonic, Tails and Knuckles are later given access to the Hidden Palace Zone. Upon entering the zone with all 7 Chaos Emeralds, the 7 gems will fly up and power up into 7 larger Super Emeralds. The Hidden Palace also contains the Master Emerald. The new Super Emeralds are located in 7 new Special Stages.
Sonic SpinballMega Drive version and Game Gear/Master System versions of Sonic Spinball feature a record total of 16 Chaos Emeralds. They are scattered across the levels in the game, and require doing certain precision pinball tricks to obtain. They are all blue, and are similarly shaped to the Emeralds in Sonic 2 (see above).
As with most other games, the Emeralds are central to the plot. The US Mega Drive manual states:
The... excessive quantity of these Spinball emeralds, along with their uniform blue colouration, and apparent absence of an associated Super transformation, leads some fans to speculate that these shouldn't be considered real Chaos Emeralds at all. It's also interesting to note that Sonic Spinball is actually set in the Archie Sonic the Hedgehog comic's world of Mobius, which could explain the overabundance of Chaos Emeralds in Sonic Spinball; in the comics of the day the Chaos Emeralds were incredibly numerous and all green colored (Why they are blue in Spinball is a mystery).
Sonic 3D Flickies' IslandSonic 2, where Sonic had to once again collect rings and avoid mines.
Sonic AdventureSonic Adventure, the Chaos Emeralds were revealed to have both Positive and Negative energies. The Emeralds were collected by Dr. Eggman to feed Chaos, the "God Of Destruction". Chaos would power up greatly and change form when being fed a Chaos Emerald and when he has been fed all 7, he could become powerful enough to destroy the world. With the Chaos' power, Eggman would create his dream-city: EggmanLand. After transforming into Perfect Chaos with all 7 Chaos Emeralds and destroying Station Square, Perfect Chaos fully absorbed the Negative Energy in the gems, and betrayed Eggman, leaving only the Positive left. Sonic then used the Chaos Emerald's Positive Energy to transform into Super Sonic and defeat Perfect Chaos. It is during this game that the color of the emeralds is suggested to be linked to the "negative" energy and their size to the "positive" energy, as the emeralds become colorless after Chaos absorbs the negative energy, but do not dissappear like in later installments until Sonic absorbs the positive energy. However, this idea is not revisted in later titles (where the emeralds simply dissappear altogether when someone goes to their Super State), so no definite conformation is available.
Sonic Adventure 2
In Sonic Adventure 2, Sonic and Shadow had the ability to use the Chaos Emeralds to perform "Chaos Control": alter time and space to teleport or freeze time. Shadow collected the Emeralds for Dr. Eggman to power the ARK, at which point Biolizard took over the station. It became FinalHazard, and then Sonic and Shadow had to team up and use the Chaos Emeralds to become Super Sonic and Super Shadow and defeat FinalHazard. Shadow eventually used a superpowered Chaos Control.
In Sonic Heroes, the Special Stages returned and collecting all the Emeralds and completing all 4 team's stories unlocked the last story against Metal Sonic, who used the numerous character's data to become a dragon beast in the fight Metal Madness. In the final fight, Sonic becomes Super, and transforms Tails and Knuckles to become Team Super to defeat the Metal Overlord.
Sonic the Hedgehog (2006)
In Sonic 06, the Chaos Emeralds are shown to not only be able to warp time and space in the present, but they can warp through time and space to different time periods altogether. This happens with numerous characters at many points throughout the game, using the power of the Emeralds to warp to different areas across the time stream. The Emeralds are shown here to also have the power to revive, as Sonic is revived from death using the power of the Emeralds. By the end of the game, the final boss Solaris exists in all three time periods simultaneously, and Super Sonic, Super Shadow, and now Super Silver have to be in the past, present and future at the same time to fight it. All this never truly happened however, as all this is erased from the timeline at the end of the game.
Shadow the Hedgehog
In Shadow the Hedgehog, the Chaos Emeralds become story elements once again, but this time Shadow is trying to find the Emeralds to discover his past. In the end, it turns out that Black Doom was using him to get the Emeralds so he could use Chaos Control on the Black Comet to get it to the planet's surface. Shadow then goes Super to stop him and destroy the Black Comet.
In Sonic Unleashed, it is learned that the Chaos Emeralds have the power to keep the planet together in one piece. By restoring the Emeralds to their own alters in each Gaia Temple, the Emerald restores its energy and reconnects a chunk of the planet. Sonic later uses the Emeralds to go Super and defeat Perfect Dark Gaia.
Sonic the Hedgehog 4
Sonic the Hedgehog 4 returns to the Giant Rings to open the Special Stages. These Special Stages, like Sonic 1, are spinning mazes which you need to navigate to earn the Chaos Emeralds. Getting all the Emeralds allow you to be Super in the main stages, and unlocks the true ending after beating the game.
In Sonic Colors, the Chaos Emeralds are unlocked outside of the main story, the first time it can be done in a 3D Sonic game. They are unlocked by earning Red Rings in the main stages, which will unlock new stages in Game Land. Beating all stages in each section of Game Land (1-1, 1-2, and 1-3 as an example) will earn the player a Chaos Emerald. Getting all 7 Emeralds will unlock a Mode in the Options Menu to play as Super Sonic in the main stages (if you play in Super Sonic mode you can't use the Colour Power of the Wisps).
Sonic Generations returns to the 3D games formula with the Emeralds as a plot device. Classic and Modern Sonic collect the emeralds from each boss and the three rival battles. One emerald is obtained in the overworld after all the levels are beaten. At the end of the game, both Classic and Modern Sonic transform into their Super forms and battle against the Time Eater.
Numerous hand-held Sonic games contain or reference the Chaos Emeralds in some form.
Game Gear titles
The 8-bit hand-held Sonic games, all released for the Sega Game Gear (with some also released on the Master System) generally had 6 Chaos Emeralds, with varying colors and ways to obtain them. The games revealed little to nothing about the emeralds, most centering around classic Sonic gameplay combined with new features and gimmicks. None of the Game Gear titles included any form of super transformation.
Sonic 1 8-bit
The Game Gear and Master System versions of Sonic 1 had the Emeralds scattered in certain acts of each zone; the Bonus Stage of the game was used to obtain rings, score, and continues. All 6 Emeralds are light blue in this game, due to hardware limitations. If all of the Emeralds are collected, Sonic holds them up at the end of the game, where they circle around the island clearing it of all of Robotnik's pollution. The manual states:
Sonic 2 8-bitversions of Sonic 2 are similar to Sonic 1: There are only 6 Emeralds, they are all different colors though, and are hidden in the zones. Collecting them all allows access to the last level, Crystal Egg Zone. The manual states:
Sonic ChaosSonic Chaos and beyond, the Emeralds would be found in special stages like in the 16-bit games. Once Sonic collects 100 rings in a stage, he is automatically sent to the special stage, as opposed to getting an extra life (once Sonic has all the Chaos Emeralds, though, collecting 100 rings will achieve the classic effect). Once inside, Sonic must find and collect the Chaos Emerald hidden somewhere in the stage, and may obtain rings along the way. Tails cannot enter the special stage, and hence cannot collect any Chaos Emeralds (he merely gets a 1-up when he collects 100 rings). Robotnik holds the 6th and final emerald, and obtaining all 6 leads to the Good Ending.
Sonic Triple TroubleSonic Triple Trouble has 6 Chaos Emeralds, the first five hidden in the special stage, with the final one held by Robotnik. Special stages alternate between a platforming stage and an airplane stage. At the end of either, you are warped to an arena where you must defeat Nack to obtain the emerald.
Sonic Drift series
Sonic Drift uses the Chaos Emeralds to denote the difficulty of the Chaos Grand Prix racing competitions. They are Green, Yellow, and Red, in order from easiest to hardest. The sequel, Sonic Drift 2, uses them in the same manner, except they are Purple, White, and Blue. They appear similar to the Emeralds from Sonic 2 and even moreso to Sonic Spinball.
The 5 Chaos Emeralds in Sonic Blast are obtained in a mixture of older games: enter the large ring in the stage (as in Sonic 3) and collect enough rings (as in Sonic 2). However, you are only allowed to enter a single stage per level; if you enter another ring you'll be taken to the same stage only able to win an extra life.
If all 5 are obtained, Sonic or Knuckles gets to fight an extra final boss and see the Good Ending.
In Tails Adventure for the Game Gear, there are 6 Chaos Emeralds, scattered throughout the levels. They are colored, in order of appearance, red, green, purple, white, blue, and yellow. Each increases your maximum rings by 10. When all 6 are obtained, maximum rings goes up to 99.
In Sonic Labyrinth Sonic is awarded a Chaos Emerald upon successful completion of each of the first 5 stages of the game. The 6th emerald is hidden within a special stage accessed in Labyrinth of the Sea, Act 3. Obtaining all 6 Emeralds unlocks the Good Ending.
Game Boy Advance
Sonic Advance Series
In the three Sonic Advance games, the seven Chaos Emeralds are all hidden in special stages. Collecting all seven unlocks an "Extra Zone" where Sonic transforms into Super Sonic and battles a final boss.
Emerl, the legendary Gizoid, draws his power from Chaos Emeralds. As he gains more and more emeralds, his speech and personality become more and more developed and his battle abilities grow stronger and stronger.
Chaos Emeralds are also featured in the Sonic Rush series, Sonic Chronicles: The Dark Brotherhood, and the DS Version of Sonic Colours. They like the Advance series unlock a Last Story, and have a final boss to fight.
Adventures of Sonic the Hedgehog
In Adventures of Sonic the Hedgehog, there are only four Chaos Emeralds. Unlike the game continuity, each of the emeralds provide their user with a different power. From top left on the image to the right: invisibility, invincibility, immortality, and "the power of life itself": the ability to bring objects, particularly Robotnik's robots, to life. For some reason, each of these emeralds only exist in the past, hence Robotnik and Sonic had to travel back in time to collect them. When Robotnik used all four emeralds at the same time, he grew to gigantic proportions and was granted god-like powers.
Chaos emeralds in Sonic Underground are green stones filled with unlimited chaos energy. There is no limit to the number of emeralds in the SoUND universe, but they are very rare - only two chaos emeralds made appearances through its entire 40 episode run. Knuckles the Echidna uses a chaos emerald to make the floating island float, which Sleet and Dingo attempted to steal for Robotnik. The other chaos emerald was found by Robotnik and used to power his flying fortress.