Unknown Gems

"Unknown Gems" are Gems who have been visually shown through holograms, drawings, or other methods. They, or their gemstones, 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.

"The Answer"
There are many unknown silhouette gems in The Answer in Garnet's flashback.

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

Trivia

 * The unnamed Gem warrior from "Sworn to the Sword" is voiced by Michaela Dietz. This may imply she is an Amethyst or some type of Quartz.