Just for information, I only watched the movie and a few episodes so sorry if Im incorrect at some points.

Moonstone is the sane kind of gem as Spinel and is close friends with her.

Formerly an entertainer, when Spinel left her side to be with the Diamonds, Moonstone was heartbroken and desired to destroy Steven and the Diamonds.

She changed form like how Spinel changed from a goofy and fun-loving person to an evil and slightly insane maniac (no offense).

Zircon is a Spinel-Pearl hybrid who formerly worked for Yellow Diamond. She was very energetic and sparky.

But because of trauma from wars and battles, she ran away and changed into a dark gem that hates anyone she comes across.