Gems as a species are characterized by the gemstone each Gem has embedded somewhere on their bodies, which seems to be analogous to a brain or CPU.
Gems' physical forms are projections from their gemstone, and Gems may change their physical form at will (although shapeshifting is not permanent and Gems will return to their "default" form eventually), with their only constant feature being their gemstone and color palette. All Gems are inherently sexless and the species lacks sexual dimorphism, although Gems' physical forms typically have characteristics associated with human femininity, with the only exception thus far being Steven, due to his half-human parentage.
Gems have numerous abilities, which can vary from Gem to Gem, including weapon summoning, fusion, the power to 'store' material in their gemstones, and bubble creation. Lapis Lazuli has no true weapon but instead possesses hydrokinesis. Similar to this is Pearl's Psammokinesis. All Gems with their own weapon have the ability to direct energy through their weapons, such as Garnet using her gauntlets to charge Greg's van, Pearl firing an energy blast from the tip of her spear, and Amethyst sending bursts of energy through her whip or whips.
Gems are functionally immortal and do not require food or sleep (although Gems can still eat and sleep if they so desire). The only way to permanently destroy or "kill" a Gem is to destroy their gemstone. If a Gem's physical form is badly injured, they will retreat into their gemstone to regenerate. Regeneration is special in that Gems can make permanent changes to their physical form while regenerating. The length of the regeneration process can range from minutes to weeks, but a "rushed" regeneration can suffer from physical deformities (such as Amethyst in "Reformed"). Regeneration can be prevented if a gemstone is bubbled or otherwise constricted, such as within a mirror or a pillow.
If a Gem's gemstone is damaged, their physical form will deteriorate and "glitch". Gem Shards, the remnants of a shattered gemstone, possess a "powerful partial consciousness" which can grant sentience to their containers and have been used to create drone soldiers.
Gem Shards have also been forced to fuse together over time to form a Cluster Gem.
The Homeworld Gems are Gems currently living on the Gem Homeworld. Historically, Homeworld Gems wished to exploit Earth and established a foothold on the planet, by which they used to create new Gems, known as a Kindergarten. The "Homeworld Gems" are a hostile military force that invades planets, bleeds their natural resources dry, and "leaves them for dead."
The Crystal Gems are a group of rebels who chose to join Rose Quartz in the fight against Homeworld in order to defend Earth. The only Crystal Gems that remain are Garnet (the fusion of Ruby and Sapphire), Amethyst, Pearl, and Steven, the son of Rose Quartz.
Over five thousand years prior to the start of the series, Rose Quartz organized the Crystal Gems to oppose the Homeworld Gems. The Crystal Gems won, as stated by Pearl in "Rose's Scabbard", and the Homeworld Gems were driven from Earth, leaving behind many relics and temples.
While Gems are incapable of reproduction, romantic relationships do exist in their society, such as that of Ruby and Sapphire.
The Gem Homeworld maintains a strict caste system based on gemstone type; in "Back to the Barn", Peridot describes how Pearls are created to be servants and meant to be completely subservient to their owners, and the rarity of a Peridot owning a Pearl.
In "Too Far", there seems to be a broader type of Gems called Quartz. Quartz Gems are created to be large and intimidating soldiers which includes Jasper and Amethyst.
Gems do not have genders.
Gem culture does not have a gender binary or a concept of gender.
Gems refer to each other using "she/her" pronouns, but have no particular reason for doing so beyond arbitrary convenience.
In their default state, Gems typically appear feminine by human standards, and they can use their shapeshifting abilities to deliberately assume overtly gendered forms.
Steven is an exception because he is half-human.
Stevonnie's precise canonical gender is currently unspecified, however Matt Burnett has stated that Stevonnie uses gender-neutral they / them pronouns.
It has been confirmed by Rebecca Sugar that Gems don't naturally reproduce and lack reproductive organs by default.
In the short "How are Gems made" It's explained that Gems "aren't born, they're made when some Gems from outer space came here with these big machines! They put the machines on Earth and shot this junk into the ground, and then boom, you pop right out!". In addition it's revealed that Gems are formed fully developed "being a weak baby would be a pain, so, they suck up all the good stuff from the ground so we pop out all big and strong".
In "On the Run", it is revealed that the Homeworld Gems invaded Earth several thousand years ago and used Kindergartens to create new Gems using the method described in "How are Gems made"; and that Amethyst was one such Gem. This process was very harmful to the environment to the point where, if it were allowed to continue, it would lead to "the destruction of all life on Earth", according to Pearl in "Marble Madness". Amethyst refers to all the Gems created through the process, herself included, as "parasites".
While biological reproduction is an alien concept to Gems, it is not impossible for them; while Gems can't reproduce with each other they can create Gem-hybrid offspring. Through shape-shifting, they can perfectly replicate reproductive organs of organic beings for their "human constructs" in order to reproduce.Rose Quartz is the only Gem to have done this and her son, Steven, inherited her Gemstone, without which she could not project a physical form and subsequently "died".
In "Ocean Gem", Pearl states, "All Gems aren't necessarily good." Gem Monsters are in fact either "corrupted" or "broken" Gems which, presumably, were unable to maintain humanoid form and sentience.
Fusions are capable of further fusing with Gems, as exhibited in "Coach Steven" (in which Garnet fuses Amethyst).
Fusions tend to highly exaggerate whatever personality traits are shared amongst the majority of the Gems comprising the Fusion. Sugilite seems to inherit her impulsive and irrational behavior from Ruby and Amethyst, while Sardonyx seems to get her haughty and self-absorbed personality from Sapphire and Pearl.
When Gems in a given Fusion have incompatible personalities, or have conflicting motives for fusing, and for whatever reason do not (or can not) terminate the Fusion, it results in an Unstable Fusion. When this happens, the constituent Gems risk losing touch with their individual personalities the longer the Fusion persists.
In "Coach Steven", Sugilite is the first example of Gems remaining as an unstable Fusion for too long. It can be surmised that this case arose because Ruby and Amethyst have many similarities in personality, while Sapphire's personality strongly contrasts with both. Sapphire's presence was enough to cause instability, but not enough to "outvote" Ruby and Amethyst and cancel the Fusion.
Malachite is another example. At the end of "Jail Break", Jasper is clearly trying to escape the Fusion. Either Jasper has very low willpower (not unlikely, considering her barbaric personality), or Lapis Lazuli has a great deal of mental fortitude, or a combination of both.
The majority of known Gem weapons are medieval: Garnet's gauntlets, Pearl's spear, Amethyst's whip, Steven/Rose's shield, Opal's longbow, Sugilite's flail, Jasper's crash-helmet, Ruby's gauntlet, and Sardonyx's war hammer.
The process for each Gem to summon their weapons depends on a different state of mind for each of them.
Amethyst places little effort or concentration, claiming that it "just happens" when she needs to summon her weapon.
Pearl claims that her process as being perfect and calculated, using the science of a flower petal dancing in the wind to describe how hard work and dedication can help master the properties of one's own gemstone and "perform their own dance".
Garnet claims that she summons her gauntlets by linking her mind with the energy of all existing matter, channeling the collective power of the universe through her gem(s), matching her calm and meditative temperament.
Steven's means of summoning his shield is still unknown, though it is thought to be caused when he experiences pure joy, having first activated his gem after taking a bite of his favorite ice-cream sandwich, his ability to summon his weapon can also be attributed to the feelings of love and the desire to protect.
Say Uncle reveals that Steven's powers are maternal, and that he summons his shield to protect the things he cares about. Though the episode is considered non-canon, the description of how Steven summons his shield seems to be accurate.
Ian Jones-Quartey has stated that Steven is the first Gem to use masculine pronouns.
Rebecca Sugar and Ian Jones-Quartey have stated that Gems use feminine pronouns by default.
Although Gems are genderless, in their human-like forms they are anatomically constructed similarly to human women, and referred to by the pronouns used by human women.
This is demonstrated in "Ocean Gem", as Pearl called Lapis "she."
Gem's physical bodies are capable of producing bodily fluids, and in "Coach Steven", Pearl can be observed to have scrapes on her "skin".
Pearl referred to their bodies as "human construct(s)" in "Fusion Cuisine".
Ian Jones-Quartey has stated that Gems do possess DNA, and that Rose Quartz was able to pass her traits on to Steven, although this would contradict Pearl's implication that Gems are inorganic in Alone Together.
This is explained in the Guide to the Crystal Gems, the Gem equivalent to genetic information is stored in the Gemstone , and Rose was able to integrate this information with Greg's DNA by giving up her physical form and passing her Gem along to Steven.