Green Diamond is a Diamond that was shattered and placed in the Cluster during the Rebellion. She is reformed and ruling Homeworld alongside the Diamond Authority.
Green Diamond was originally part of the Great Diamond Authority. She disliked the strict rules and disobeyed Homeworld by being kind to her Gems, supporting the Rebellion, allowing cross-Gem fusion, and befriending her Pearl, who chose the name April.
Once Pink Diamond was shattered, Green lost her closest Diamond ally. She moved to stop the Rebellion and was put on trial by Yellow for betrayal of Homeworld. Coral, her right hand Gem, defended her and Zircon was the prosecutor. Green Diamond got angry and summoned her Gem weapon, damaging but not poofing Yellow Diamond. Yellow shattered Green Diamond and she was buried in the Cluster.
Several thousand years later, Steven found two of her shards and used his spit to heal them together, allowing her to partially reawaken. As Steven continued this and Lapis and Peridot joined in the effort, Green Diamond became the second Gem from the Cluster to fully reform. She projected herself a form, but stayed as a 'Charcoal', unable to access her Diamond powers. After a series of strange visions, she found out that she was really Green Diamond, but stayed in her Charcoal form.
When Yellow Diamond came to Earth and attacked Coral, Charcoal transformed and became Green Diamond. She battled and poofed Yellow Diamond, then returned to Homeworld.
She unbubbled her Gems and is currently ruling Homeworld alongside the Diamond Authority.
When Green Diamond was a Charcoal, she was shy and quiet, mostly talking to her best friend Coral. Now, on Homeworld, with more Gems and on a place that was less holey, she is now more outgoing and shows her colors more often.
She is sympathetic to fusing, and in fact has at one point fused with Coral. She can be rather rash, as evidenced when she summoned a weapon in the middle of a court trial. (Although, Yellow Diamond did insult her Coral.)
Steven Universe-She likes and trusts Steven, as he helped her get her form back. She gladly does things for him, including coming to see a very odd play at his request.
Lapis Lazuli-She is on friendly terms with Lapis as she aided in the reformation of the Cluster gems.
Peridot-She is on friendly terms with Peridot as she aided in the reformation of the Cluster gems.
Crystal Gems-She allied with the Crystal Gems in the war and likely has a friendly relationship with them.
Yellow Diamond-Now that Yellow Diamond is less angry, Green Diamond maintains a fragile but peaceful relationship with Yellow.
Blue Diamond-Green Diamond is more tolerant of Blue Diamond due to her less ferocious nature.
White Diamond-Green Diamond admires White Diamond because she created the other Diamonds, and White Diamond is neutral towards Green.
Pink Diamond-Green Diamond was close with Pink Diamond, being younger and therefore more understanding of Pink. The two were on opposing sides of the war, and Green was shattered prior to Pink Diamond's shattering.
Coral-Green Diamond was in love with Coral prior to the shattering of herself. Coral was shattered after Green was and the two recalled nothing but their close bond after they were reformed. They are still in love.
April-Green Diamond is very kind to her Pearl, often carrying her around on her head.
Her court-Green Diamond is friendly and kind to her court.
Sound waves-Green Diamond can shoot sound waves.
Knife-Green Diamond can summon a maroon knife.
Healing corruption-As a Diamond with a talent for music, she has the ability to reverse the Corruption.