Peridot, also known as the Gem Monster, is an off-color Peridot Homeworld gem and member of the Gem Queens.
Once a simple era 1 peridot, she was selected by the diamonds to undergo dangerous experimentation in the war against the Crystal Gems. Looking to create powerful warriors by half corrupting gems, Perdiot was subjected to the corruption and managed to keep a somewhat humanoid gem form.
However, the corruption soon discolored her gem and she began to act erratically and would often lose control of her new corrupted powers. She eventually went on a rampage throughout Homeworld, poofing and shattering dozens of gems. She was evneutlaly subdued by the diamond authority and their guards, but was rescued by Mother before she could be shattered.