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.
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. It's unknown if this applies to every Gem, should they want to have a child. Rose passed her gem, a rose quartz, to Steven.
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, their body retreats into their gems so they can regenerate a new body. The process can take weeks. There is also a chance they will have different outfits with their new body.
- It can be assumed that Crystal Gem culture is quite different from Earth culture, given the Gems confusion over seemingly everyday Earth things such as birthdays and video games.
- The Crystal Gem culture is an ancient one possible pre-existing the human race.
- Pearl points out in "Tiger Millionaire" that they are not allowed to use their powers on humans, which is the only law seen so far.
- The Gems all wear stars on their outfits, its possible stars may be an important symbol to them (or just a fashion choice).
- The Gems don't appear to get paid for their services, receive any awards, or are even acknowledged by the beings they are protecting. The citizens of Beach city are shown to be completely oblivious to their saviors existence or duties, and the gems are never offended by their ignorance or demand any rewards for what they are doing. This can conclude that the way of their culture is to protect other races, without asking anything in return.
- 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.
It has been confirmed by Rebecca Sugar that normal Gems don't have parents and that they don't naturally reproduce. Makes sense since they are an immortal race so breeding is not a priority. Rebecca Sugar also confirmed that Rose giving up her physical form to give birth to Steven was extremely unusal for a Gem.
Since they do not breed they likely do not have any form of courtship.
The existence of Steven proves that they are compatable with other species (or at the bare minimum humans).
Though the ones shown so far look and sound female, the Crystal Gems are a genderless race with Steven being the first male Gem. Since they are an immortal race, breeding is probably not a priority.
- List of known Crystal Gems and their ties to birth stones (by month). 
- All of the Crystal Gems' weapons (including Steven's shield) have the same color scheme as their gem.
- All of the gems (Excluding Steven) seem to know how to drive
- Steven is the only member of the Crystal Gems whose skin isn't the same color as his gem, though his skin still exhibits a pinkish hue.
- Amethyst is a birthstone associated with love. The stone itself is associated with Dionysus, the god of wine and intoxication. Amethyst is not a clear thinker but is fun-loving.
- Pearl is a stone associated with purity. Pearl is a mature, wise and perfectionist person.
- Garnet is a stone that protects from evil. Garnet is the most powerful and often saves the day. She is also the leader of the Crystal Gems.
- Rose Quartz is associated with love and joy. Steven is the most caring, and upbeat of the gems, and Rose Quartz is implied to have been very loving of everything.
- Their gem placement seems to correspond to their personalities, actions, and how they think.
- Amethyst's gem is over her heart; she tends to be fun loving, and often does whatever she wants.
- Pearl's gem is on her forehead; 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.
- It is revealed by Pearl in "So Many Birthdays" that gems don't age, but can still die if they are hurt badly enough and are not treated.
- This trait does not apply entirely to Steven. He seems to have the ability to age but can apparently alter his age depending on his mindset and emotions.
- While Gems can't die from food poisoning, they can still get extremely sick to the point of vomiting.
- This does not apply to Steven, a half-Human, half-Gem. In "So Many Birthdays," Steven accidentally ages to the point of near-death.
- All Gems seem to have stars located somewhere on their outfits.
- Steven: on his t-shirt.
- Garnet: on the chest area of her body suit.
- Amethyst: star-shaped holes on her leggings.
- Pearl: on her shirt.
- Opal: on the draping section of her shirt.
- Rose Quartz: star-shaped hole on the stomach area of her dress.
- Sugilite: ragged, star-shaped holes on her knees and painted on her nails.
- Even though all the Gems have the ability to shapeshift, Amethyst, Steven, and Garnet are the only ones shown demonstrating it so far (Amethyst performs the ability the most).
- 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:
- It is revealed in "Steven the Sword Fighter" that Crystal Gems retreat to their gem and lose their physical form when it is badly injured. It is unknown how long it usually takes for Crystal Gems to regenerate as Pearl's regeneration took about two weeks. Also, when Crystal Gems regenerate their bodies, they can get a new outfit as shown in the aforementioned episode.
- According to Rebecca Sugar, Gems do not need to eat or sleep, but they can enjoy doing so anyway.
- If multiple gems stay fused for too long or have an unstable personality, they can slowly lose their individuality. A prime example is the fusion gem Sugilite from the episode "Coach Steven."
- All of the Gems weapons are medieval: Garnet has gauntlets, Pearl has a spear, Amethyst has a whip, Steven has a shield, Lion has Rose's sword, Opal has a bow and arrows, and finally Sugilite has a flail.
The gems themselves seem to correspond with the Crystal Gems personalities.
- Pearl's gem, the pearl, being automatically smooth and perfect.
- Amethyst's gem, an amethyst, being coarse and rough.
- Garnet's gem, a garnet, specifically focused on raw garnets, which are mysterious with just a little bit of red peeking through hidden inside.