There is an unknown gem seen in the play Steven and Jamie put on in "Historical Friction". She doesn't appear to resemble any other fusions. It is implied that she is a fusion of some or all of the Crystal Gems. There is also a monster, it is most likely a gem monster.
A picture of this unknown fusion from the episode.
The giant gem monster that resembles the centipeetle.
An unknown, bright-blue colored gem in Garnet's flashback with a body, hair, and gemstone extremely similar to that of Lapis Lazuli. Her tear-shaped gem is on her back.
An unknown, bright-blue colored gem (6th to the left in picture) in Garnet's flashback with a body, hair, and gemstone extremely similar to that of Lapis Lazuli. Her tear-shaped gem is on her chest.
An unknown, carnation-pink colored gem (4th to the right in picture) in Garnet's flashback with "swoopy", pointed hair. Her circular gem is on her chest.
An unknown, salmon colored gem (2nd to the right in picture) in Garnet's flashback with hair in two buns. Her horizontally elongated gem is on her chest.