Unknown Gems

"Unknown Gems" are Gems who have been visually shown through holograms, drawings, or other methods. They have not been officially identified.

"Historical Friction"
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.

Comics
There are some unknown gems from the Steven Universe comics.