Gem

"Gems" are an alien race of "magical" beings that play a central role in Steven Universe. Though they mostly reside on their home planet Homeworld, a large number of Gems (including the Crystal Gems) reside on Earth.

General Description
They are sentient gemstones that can project a tangible humanoid form. They are an intergalactic race which 5,750 years ago maintained several outposts on Earth, with plans to colonize the planet, but were driven off by Rose Quartz's rebellion, leaving the Crystal Gems and the corrupted Gems as the only Gems on the planet.

Though pure Gems are referred to in feminine terms and commonly have feminine traits, Gems are a genderless race, with Steven Universe being the first male Gem to exist, as well as the first human-gem hybrid. Despite this, Gems with more masculine appearances and voices are not completely unheard of.

Appearance and Gender
Crystal Gems generally take on a feminine or androgynous human-like form, notably distinguished by the lack of gender-defining traits such as breasts. For the most part, they appear to lack external ears, and some Gem types may lack a nose or have only one eye. The forms they take are only constructs their gem creates for them, which follows restrictions set by their biological caste. The Gem's appearance varies from Gem to Gem, but their colors are mainly composed of the color palette of the gem they possess. If a Gem's physical form is destroyed, they will retreat into their gemstone in order to create a new form.

Gems are not biologically male or female, but use feminine pronouns for linguistic purposes, with Steven being the only male and possibly the only Gem with a gender identity at all due to his human half. Fusions between Steven and anyone who uses she/her pronouns apparently uses they/them pronouns.

History
All Gems have superhuman powers such as shapeshifting, Gem bubbling, weapon summoning or elemental control, and Gem fusion. For thousands of years, the Crystal Gems have used these abilities to defend Earth from magical threats. One of the Crystal Gems (and former leader), Rose Quartz, gave up her physical form to give birth to a half-human child, Steven Quartz Universe. Garnet, Amethyst, and Pearl consider Steven an important member of the Crystal Gems, and the three have taken it upon themselves to raise and teach him how to use his powers.

In several episodes, Pearl references that there are/were more of their kind, but never elaborated on the absence of other Gems on Earth, implying that they may have died or disappeared. In the episode "Ocean Gem," it is revealed that there are in fact other Gems, and that the monsters that the Crystal Gems fight are former Gems that had been corrupted.

As revealed in "Space Race", there are more Gems on different Gem-controlled planets. The Galaxy Warp was formerly used to travel off of Earth to different Gem planets, but is currently broken. This reveals why only the Crystal Gems reside on Earth.

Culture

 * Pearl points out in "Tiger Millionaire" that, as Crystal Gems, they are not allowed to use their powers on humans. Although, that was broken when a human named Ronaldo kidnapped Steven.
 * Whereas the Crystal Gems incorporate stars into their clothing, Homeworld Gems use diamonds.
 * The typical way for a Homeworld Gem to address a Diamond is "My Diamond" while either bowing or saluting.
 * Sapphires are addressed as "Your Clarity" or "Your Grand Clarity" by lower castes.

Biology
Gems seem to be mineral-based lifeforms due to primarily being living gemstones. A gem's gemstone is positioned somewhere on their physical form (solid light hologram/projection), such as the forehead, palms, chest, back or navel. Gems are also capable of shapeshifting, but this is known to consume a lot of energy and leave a Gem exhausted when done for too long. They are functionally immortal and cannot die from old age, but can die by other means, such as being shattered. If a Gem is severely injured, their body dissipates in a burst of dust or smoke, leaving behind their gemstone, which is vulnerable and unable to move until the gem finishes regenerating. In addition, if a Gem's gemstone is damaged, their physical form will become distorted. Gems also do not need to eat or sleep, but are nonetheless capable of doing so, and they can get physically exhausted.

Gems are also capable of fusing with each other in order to become a gigantic version that can comprise of two or more gems in number. The fusion forms often have more than one pair of arms (or legs in Malachite's case), depending on the number of gems the fusion is made of and the compatibility of the individuals, though this seems to only apply to different types of gems, as fusions of Gems of the same type do not have additional limbs.

New gems were created from facilities called Kindergartens. While biological reproduction is an alien concept to Gems, it is not impossible for them; while Gems can't reproduce with each other, at least one (Rose Quartz) was able to create Gem-hybrid offspring by shapeshifting to create a reproductive system. Her son, Steven, inherited her Gemstone, without which she could not project a physical form and subsequently "died".

Gems are unaffected by changes in gravity, and do not need to breathe, but do appear to have lungs for speaking purposes, and are not immune to gasping for breath.

While Gems are normally sentient, they can be become feral and animalistic in a process known as "corruption". Nephrite, for instance, was formerly a normal Gem, but became a centipede-like beast when corrupted. Corruption also seems to function like a disease, and can spread though fusion, as seen when Jasper fused with a corrupted Gem she had captured to battle Smoky Quartz and eventually became corrupted as a result. Corruption is not known to occur naturally, and originated from an attack on Earth which was intended to kill every Gem on the planet but failed.

Fusions
It is revealed in "Giant Woman" that the Gems are able to fuse together into even more powerful Crystal Gems. Here are the ones shown so far:
 * Ruby + Sapphire = Garnet
 * Amethyst + Pearl = Opal
 * Garnet + Amethyst = Sugilite
 * Garnet + Pearl = Sardonyx
 * Steven + Connie = Stevonnie
 * Steven + Amethyst = Smoky Quartz
 * Steven + Pearl = Rainbow Quartz 2.0
 * Steven + Garnet = Sunstone
 * Steven + Greg = Steg
 * Rose Quartz + Pearl = Rainbow Quartz
 * Pearl + Ruby = Rhodonite
 * 6 unknown gems = Fluorite
 * Jasper + Lapis Lazuli = Malachite
 * Jasper + Ocean Jasper = Zebra Jasper
 * Left Topaz + Right Topaz = Topaz
 * Garnet + Amethyst + Pearl = Alexandrite
 * Garnet + Amethyst + Pearl + Rose Quartz or Steven = Obsidian.

Gems cannot fuse with humans, as shown in "We Need to Talk". However, Gem-human hybrids can fuse with humans, as Steven did with Connie to form Stevonnie. Hybrids can fuse with pure Gems too, as Steven formed Smoky Quartz with Amethyst, Rainbow Quartz 2.0 with Pearl, and Sunstone with Garnet.

Even feral, corrupted gems are capable of fusion, as Jasper fused with the Snow Monster to form Zebra Jasper.

A secondary form of Fusion is known as Force-fusions or Cluster Gems, first revealed in "Keeping it Together". These are very unstable, and attack non-cluster Gems to find their missing shards and become "whole" again.