Green Diamond is the fusion between Yellow Diamond and Blue Diamond. They technically don't appear in the actual series, but had been mentioned twice by other characters.
In the three-part-finale of My Little Universe: Season 6, White Diamond mentions them being the new ruler of Homeworld in the upcoming Era 3. Rather than a fusion though, she planned to bring her out by hybridizing Yellow and Blue together, which didn't happen.
Green Diamond is brought up again in "I Have Spoken!", as The Spirit brings them up during the trial against White Diamond.