The Chaos Emeralds are seven powerful artifacts from Mobius, Sonic's world. The exact details about them are shrouded in mystery, but they have been in existence for at least 4000 years and used by many an ancient civilizations, such as the Knuckles and Nocturnus Clans, and even extraterrestrial beings, such as the Black Arms, for the purposes of powering machinery or even themselves. They possess both positive and negative energies, which are accessed based upon the thoughts of the one who holds them and are considered more powerful than anything in Sonic's dimension, save the Master Emerald, which serves as the "master" over the Chaos Emeralds' power.
The Chaos Emeralds are seven colored gems, with colors ranging between blue, red, yellow, purple, white, cyan and green, that house unimaginable power. All seven are bound by and regulated by the Master Emerald, a similar artifact of a much greater size. Their history is mostly affiliated with the Knuckles Clan, who were wiped out by the water god Chaos long ago admist the war with a rival clan, specifically the Nocturnus Clan. Due to their power and what threat they could pose, they are often sought out by heroes and foes in attempts to achieve ultimate power or save the world.
The Green Chaos Emerald in particular has the usual distinction of being the last Chaos Emerald to be found, as it is either the furthest away, the most hidden, within the possession of another person, namely villains, or simply because it is the last one left to find. Coincidentally, the Green Emerald shares its coloring with the Master Emerald.
The Blue Chaos Emerald was first seen in the Donut Plains, when a wandering Goomba happened to come across it. Seeing nobody around, the Goomba took the Emerald for himself, planning to give it to Goombella in exchange for a date. However, at that moment, Metallix appeared and demanded that the Emerald be given to him. The Goomba stood there scared, and Metallix killed him with a blast of energy, taking the Emerald for himself.