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"
In Steven and Jamie's play, "Beach City or Bust", props for a Crystal Gem fusion and Corrupted Gem Monster can be seen. Not strongly resembling any other fusions, this is a fusion between some (or all) of the Crystal Gems. This fusion fights off a Corrupted Gem resembling both the Centipeetle Mother and the Big Bird.

"When It Rains"
There are holograms of many unknown Cluster Gems in The Prime Kindergarten Control Room in "When It Rains".

"The Answer"
There are many unknown Gems (represented in silhouettes during Garnet's flashback) in "The Answer".

"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. Also, a large type of Gem, presumably Quartz or Topaz can be seen.

"That Will Be All"
Several Rose Quartzes appear bubbled in a room of Pink Diamond's Zoo.

They are also mentioned on "The Trial" by the defending Zircon who states that they were bubbled during The Rebellion before Pink Diamond's death.

"The Trial"
The defending Zircon states that Pink Diamond's entourage was composed of several Agates, Sapphires, and her Pearl.

"Off Colors"
Morganite, an unknown Gem commander, is mentioned by Rhodonite. She was in charge of the two halves of Rhodonite (a Pearl and a Ruby) before she caught them fusing and replaced them.

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 Unknown Quartz Warrior in "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.