Unknown Gems

Unknown Gems are Gems who have been visually shown through holograms, drawings, named, 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 does not 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 Gems (simply more than the total count of all the unknown gems in the previous episodes) in The Answer in Garnet's flashback.

"Same Old World"
There are a few unknown Gems featured in Lapis' flashback.

"Monster Reunion"
There are a few unknown Gems featured in Centipeetle's flashback.

"Bismuth"
These are the "other Crystal Gems" who were mentioned by Bismuth, except for Nephrite, which is most likely a Homeworld Gem. They appear to be her close friends. Their statuses are unknown.
 * "Crazy Lace" (Crystal Gem, mentioned by Bismuth)
 * "Biggs" (Crystal Gem, mentioned by Bismuth)
 * "Snowflake" (Crystal Gem, mentioned by Bismuth)
 * Nephrite (Homeworld ship pilot, mentioned by Bismuth to Pearl)

"Beta"
Many Corrupted Gems are seen trapped in cages in the Beta Kindergarten. Their bodies are hidden by shadows and only their glowing yellow eyes can be seen.

"Earthlings"
Another Corrupted Gem can be seen in a cage. Only its two eyes and large six clawed hand can be seen.

"Adventures in Light Distortion"
When Steven is adjusting the Roaming Eye's form calibration system for the Crystal Gems, a small type of Gem can briefly be seen on the panel, which subsequently alters the Crystal Gems' forms to be small.

"That Will Be All"
There are numerous bubbled Rose Quartz-type Gems in Pink Diamond's Zoo sanctuary.

Attack the Light
There is an unknown Gem from Steven Universe: Attack the Light.

Steven Universe 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.
 * The Gems mentioned by Bismuth all correspond to real-life minerals. "Crazy Lace" is a variety of agate. "Biggs" is a variety of jasper. "Snowflake" is a type of obsidian. "Nephrite" is one of the minerals called jade.