Crystal Gems

The Crystal Gems are the main heroes in Steven Universe. They are a species of magical beings with gems affixed to their bodies who play the role as guardians of protecting humanity. They all have gems that grant them special abilities, but Steven does not have control of his gem.

Description
All of the Crystal Gems have the ability to summon weapons and shapeshift, as well as the ability to fuse with each other like Opal who appeared in the episode "Giant Woman," and Sugilite who made her debut in the episode "Coach Steven." It appears that they have specialized abilities as well, so to bring Steven into the world.

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 (though they can still be killed in battle). It is possible that there are more Crystal Gems because Pearl mentioned that there were many Gems on Earth in the past. Crystal Gems are also immune to dying from food poisoning. Also, it has been recently revealed then when gems get badly injured/killed, they will disappear and retreat into their gems so they can regenerate a new body. The process can take weeks or longer. There is also a chance they will have different outfits with their new body. It was also reveled in "Indirect kiss" that if their gem is damaged, their body will go through horrific deformities, so if one wanted to kill them they would have to destroy the gem.

It is possible that the Gems themselves are just gems, they could be sentient crystals infused with energy and may take on human form for the sake of convenience.

History
For thousands of years, the Crystal Gems protected both Beach City and the universe. They defended Earth from many different creatures. They have superhuman powers like transformation, bubble shields, weapon summoning, and Gem combination. In many episodes, Pearl references that there were many other Gems, but they may've died or disappeared. One of the Gems, who could've been the leader, Rose Quartz, gave up her physical form to give birth to Steven. As of now, there are four Gems, three female Gems, and one male Gem. It appears that all the Crystal Gems are named after the stone that gives them their powers, given that the names Crystal Gem or Gem can refer to their kind or said stone, with the exception of Steven. Steven's Gem is a Rose Quartz which he inherited from his mother, whose name was Rose Quartz, giving more proof to this point.

Corrupted and broken Crystal Gems
In Ocean Gem Pearl reveals that some of the Crystal Gems are not good.

It was revealed that all the monsters fought to the point of Mirror Gem were in fact Crystal gems that became corrupted or damaged.

When the Crystal Gems came to Earth, Some of them believed their comrads only cared about humans and not themselves. When they tried to leave earth, they were stopped and became broken gems, some of them became uncontrolled monsters.

It can be speculated that the reason there are no more gems is because they all turned into monsters and were killed, captured, or on the loose.

Gender
Though most of the ones shown so far look and sound female, the Crystal Gems are a genderless race with Steven being the first male Gem.

Members

 * Garnet
 * Pearl
 * Amethyst
 * Steven Quartz Universe
 * Rose Quartz (former member, deceased)
 * Lapis Lazuli (other known gem)
 * Peridot (other known gem)
 * Opal (fusion form of Pearl and Amethyst)
 * Suglite (fusion form of Garnet and Amethyst)
 * Alexandrite (fusion form of Garnet, Pearl and Amethyst)
 * Stevonnie (fusion form of Steven and Connie)

Trivia

 * In the "Pilot" and "Cheeseburger Backpack", it is implied that the Crystal Gems are over a hundred years old. It is possible that there are more Crystal Gems because Pearl mentioned that there were many Gems on Earth in the past.
 * All of the Crystal Gem's weapons (including Steven's shield) have the same color scheme as their gem.
 * Steven is the only member of the Crystal Gems whose skin isn't the same color as his gem.
 * Their gem placement seems to corespond to their personailities, actions, and how they think. Amythest's gem is over her heart; she tends to respond to whatever her heart tells her and does whatever she wants. Pearl's gem is on her forhead for her brain; she tends to overthink things and be a perfectionist. Garnet's gems are on her hands; she tends to not talk often and respond through action instead. Steven's gem is over his stomach; he tends to go with his gut instinct and is often hungry. Their summoning of the gems tends to also match their gem placement.