Sonic Adventure 2Edit
In these series, they are pieces of the Master Emerald that are trying to be recovered by Knuckles the Echidna and Rouge the Bat. They end up in Wild Canyon, Dry Lagoon, Pumpkin Hill, and Aquatic Mine. Eventually, Knuckles lose his pieces, but gets them back as well as Rouge's, restoring the Master Emerald.
They are pieces of Chaos Emeralds that Eggman places in Chaos Gamma and his E-121 Phi robots. They are all destroyed and recovered by Emerl and used to become Ultimate Emerl. They are broken once again when Emerl is destroyed.
They only act as pieces of the Master Emerald, when Chaos is awakened by Eggman destroying the Master Emerald. Knuckles restores the Master Emerald and later in Season 3, it is shattered again when the Master Emerald releases all of it's power to destroy Final Nova.