Gems

Gems are extra-terrestrial intelligent creatures, with gems affixed to their bodies. These gems give them abilities, and there is often a relationship between the gem and the entity.

Known Gems

 * Lapis Lazuli
 * Watermelon Tourmaline

Description
All Gems have the ability to summon weapons, shapeshift, as well as the ability to fuse with each other to create more powerful Gems such as Opal and Sugilite. It appears that there are specialized abilities for each individual Gem as well, such as Garnet's abilities to swim in lava and generate electricity.

In the "Pilot" and "Cheeseburger Backpack," it is implied that the Crystal Gems are over a hundred years old. In "So Many Birthdays" it was full-on unveiled that the Gems are immortal and cannot die of old age and food poisoning (though they can still be killed in battle, presumably if their gem is destroyed). It has also been revealed that when a Gem's body get badly injured, they will disappear and retreat into their gems to regenerate a new physical form. The process can take weeks or longer. Regeneration gives Gems the opportunity to alter their default physical appearance, this including their outfits. It was revealed in "An Indirect Kiss" that if their gem is damaged, their body will go through horrific deformities.

It is implied that Gems themselves could be sentient crystals infused with energy that take on humanoid forms merely for the sake of convenience.

Culture

 * Gems use teleportation devices called warp pads.
 * Gem culture is quite different from Earth culture, given the Crystal Gems' confusion over seemingly everyday Earth things such as birthdays and video games.
 * The Gem culture is an ancient one, possibly pre-existing the human race.
 * Pearl points out in "Tiger Millionaire" that, as Crystal Gems, they are not allowed to use their powers on humans.
 * The crystal gems is a group of gems that protect humans on earth.
 * The Crystal 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.

Broken and Corrupted Gems
In "Ocean Gem'' Pearl reveals that not all Gems are good.

It was revealed that all the monsters fought to the point of "Mirror Gem '' were in fact Gems that became corrupted or damaged. It is unknown what causes them to be like this.

It is possible that the Gems on Earth that have become monsters had the same goal as the Crystal Gems: Protecting humanity, but eventually lost their way and became corrupted.

The reason there are no other known Gems is because they all either turned into monsters and were killed, captured, and contained by the Crystal Gems, or are just not on Earth.

As of "Ocean Gem ", five 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 ("Beach Party", formerly Watermelon Tourmaline).
 * Ice Monster ("Monster Buddies")

Reproduction
It has been confirmed by Rebecca Sugar that normal Gems don't have parents and that they don't naturally reproduce. Rebecca Sugar also confirmed that Rose giving up her physical form to give birth to Steven was extremely unusual for a Gem. This makes sense, as they are an immortal race and therefore breeding is not a priority. Gems also lack reproductive organs. Matt Burnett, however, has stated that this should not imply that romantic relationships can't exist among Gems.

The existence of Steven and the fact that Gems do not have a reproductive system of any sort proves that Gems do not produce offspring by any biological means.

Gender
Gems are not biologically male or female, or have female and male designations amongst themselves. Though, the ones shown so far appear feminine and use feminine pronouns, with Steven being the first male due to his human half. The Gems' usage of gendered pronouns with one another does not deter the fact that they are all just Gems, first and foremost. It does appear they can adopt either a feminine or masculine personality, despite not having sexes. Rebecca Sugar had stated that Gems in general have a 50/50 chance of using pronouns.