Gems

"Gems" are an extra-terrestrial species of magical beings that play a central role in "Steven Universe". Their origins, history, and culture are at this point not well understood, although numerous small hints throughout the series imply the Gems are an interstellar race which at one point maintained several outposts on Earth, but suddenly fled and left many of their artifacts to ruin, with the only Gems currently still on Earth being the Crystal Gems.

Regular Gems

 * Pearl
 * Amethyst
 * Lapis Lazuli
 * Rose Quartz
 * Peridot
 * Sapphire
 * Ruby
 * Jasper
 * Yellow Diamond

Gem Fusions

 * Garnet
 * Opal
 * Sugilite
 * Alexandrite
 * Malachite

Half-Gems

 * Steven

Half-Gem Fusions

 * Stevonnie

Description
Gems are characterized by the gemstone which every Gem has embedded somewhere on their body. Each Gem shown thus far is unique and their gemstones seem to be analogous to the brain or a CPU. Generally, Gems take on the form of feminine humanoids, but appearance varies from Gem to Gem with their colors palettes mirroring the color palettes of their gemstone (e.g. Garnet is mainly red, Amethyst is mainly purple/violet). Gems do, however, possess the ability to shapeshift, with their only constant physical feature being their gemstone which will manifest on any form they take. Because they can change their physical form at will, all Gems are inherently genderless (although almost every Gem up to this point uses female pronouns) with the only exception thus far being Steven due to his half human parentage. While Gems are capable of eating or sleeping, they do not require food or rest and appear to be completely self sufficient.

Gems have numerous other abilities such as summoning weapons through their gemstones (but this may not be universal as Lapis Lazuli and Peridot have not yet been observed to have weapons), the ability to fuse with each other to create more powerful Gems such as Opal and Sugilite, and the power to 'store' material in their gemstones (not counting their weapons) such as the mirror containing Lapis Lazuli from "Ocean Gem" and the police tape from "Lars and the Cool Kids", both of which Pearl withdrew from her gem. On top of this Individual Gems may have specialized abilities as well, such as Garnet's abilities to swim in lava and see into the future, or Lapis Lazuli's ability to manipulate water, and Pearl's ability to manipulate sand.

"So Many Birthdays" revealed that Gems are functionally immortal and cannot die from aging or food poisoning, although Gems can still be killed if their Gemstone is destroyed. It has also been revealed that when a Gems' physical body is badly injured, they will retreat into their gemstone to regenerate (this process can take weeks or longer). In "An Indirect Kiss" it was shown that if their gemstone is damaged, their body may go through glitch like deformities.

It is also noteworthy that a Gems' gemstones can be used as a power source even if it's damaged, as in the case of Lapis Lazuli, whose cracked gem was used to power a magic mirror that could record and show the past, but she was unable to escape or manifest a physical form, with her only means of communication through the mirror. Similar to this are Gem Shards which appear to be the remnants of a shattered gemstone and, as described by Pearl, were used by other Gems to create "drones" and could grant rudimentary sentience to their containers.

In "Jail Break", it was shown that Fusion Gems can also fuse to make other fusion gems. Garnet is a fusion gem of Ruby and Sapphire, but she can also create other fusions such as Alexandrite and Sugilite.

Culture

 * There are currently two known groups of Gems: The Homeworld Gems and the Crystal Gems. Roughly six thousand years ago, agents of the Homeworld had plotted to exploit Earth for unknown reasons, and already had a foothold on the planet which they used to create new Gems, known as Kindergarten. This group has been shown to be part of the Homeworld by Peridot's connection to the Homeworld. Rose Quartz, however, wanted to protect the life on Earth from this and created the Crystal Gems for the purpose of defending earth from Gem based threats. In the time between the founding of the Gems and Steven's birth, there was a war between those who sided with Rose Quartz and the Homeworld Gems, such as Jasper. The Crystal Gems won, as stated by Pearl in Rose's Scabbard, and the Homeworld Gems were driven from Earth.
 * Gem civilization is very advanced and capable of creating spaceships, pocket dimensions, and teleporters (among others).
 * Gem culture is quite different from human culture, given the Crystal Gems' confusion over seemingly everyday Human cultural practices such as birthdays and video games.
 * Gems (excluding Steven) do not use the human gender binary (none of them are male or female.)   They are either genderless or a non-binary gender.
 * The Gem culture is an ancient one, pre-dating the human race.
 * Gems have their own written language which is inscribed on many of their temples and which was first seen in Steven's room in the pilot.
 * Examples of Gem architecture can be seen in the form of the structures they left behind on Earth such as the Lunar Spire, Communications Hub, and the Galaxy Warp among others.
 * While there has never been mention of Gems practicing any form of spiritualism, the existence of the Moon Goddess Statue along with the cloud, underwater, and Pyramid temples imply some sort of belief system.
 * Gems most likely do have some form of military force as in "Frybo" Pearl explains the history of the Gem shards being used to create "drone soldiers".
 * While Gems are incapable of reproduction, romantic relationships do exist in their society (Such as Ruby and Sapphire, who's relationship has been confirmed to be romantic by Joe Johnston.)

Broken and Corrupted Gems
In "Ocean Gem", Pearl and Amethyst explain that "not all Gems are good" and that all of the Gem Monsters that the Crystal Gems fought up until "Mirror Gem" were in fact either corrupted or broken Gems which presumably either cracked their gems like Amethyst did in "An Indirect Kiss" or through some other means where unable to maintain humanoid form and sentience. It is possible that the Gems on Earth that have become monsters used to be followers of Rose Quartz, but eventually lost their way and became corrupted.

As of "Island Adventure", the monsters that have been previously encountered are corrupted Gems, they are the following:
 * Centipeetle Mother ("Gem Glow")
 * Worm Monster ("Bubble Buddies")
 * Gem Cave Creature ("Arcade Mania")
 * Giant Pufferfish (confirmed to be Water Tourmaline) ("Beach Party")
 * Ice Monster ("Monster Buddies")
 * Invisible Gem ("Island Adventure")
 * Flying Monster ("Giant Woman")
 * Lighthouse Gem Monster ("Horror Club")
 * The Geo-beetles of Heaven & Earth ("Giant Woman")

Reproduction
It has been confirmed by Rebecca Sugar that Gems don't naturally reproduce and lack reproductive organs. Sugar also confirmed that Rose giving up her physical form to give birth to Steven was extremely unusual for a Gem.

In "On the Run" it is revealed that Gems from the Gem Homeworld used a canyon on Earth, referred to as "Kindergarten," to create new Gems; and that Amethyst was one such Gem. The methods used to accomplish this are as of yet unknown, but it can be deduced that it most likely involved the large bacteriophage-like machines laying Gems in the holes they drill such as the hole which Amethyst claims was "Where I came from."

Trivia

 * In "Alone Together" Pearl comments that Steven may not be able to fuse because he's 'organic'. This would mean that Gems aren't carbon based life forms and are either most likely silicon based (as silicon is chemically similar to carbon) or a type of life form completely unknown to humans.
 * Gems physical bodies are capable of producing bodily fluids such as saliva, tears, etc. Furthermore, in "Coach Steven", Pearl can be observed to have scrapes on her "skin", however, being functionally immortal and seemingly autotrophic it's questionable why Gems would need such adaptations, especially since Gems' physical forms are just projections from their Gemstone.
 * Whatevenaregems.png this has not been seen, it's possible that there could be multiple gems sharing the same type of Gemstone, as Jasper refers to Pearl as "some lost, defective Pearl" in "The Return".
 * Half-Gems can fuse with humans.
 * Gem fusions can fuse with other Gems or Gem fusions, as Garnet (a fusion gem) fused with other Gems before.