Gemstones

Gemstones are the core components of any Gem. They appear as literal gemstones in various cuts, shapes, and colors located randomly on any Gem's physical form.

Description
The gemstones are, in essence, the Gem being itself. According to Pearl in "What Are Gems?", their physical bodies are merely a hologram with mass from within the gemstone, much like their weapons and outfits. Garnet states in "Steven the Sword Fighter" that if the physical body (which is just an "illusion" made of light with mass) of a Gem is severely damaged or injured, it retreats to its gemstone to regenerate and project a new body form. All Crystal Gems besides Steven have been shown to regenerate.

In contrast, any slight damage to the gemstone, such as a crack, results in a serious and often crippling injury to the Gem. It can interfere with their magical Gem powers, as Lapis Lazuli is unable to summon her water wings while her gemstone is cracked in "Ocean Gem". It also interferes with their ability to manifest/control their physical bodies as Amethyst lost more and more control of hers as her gemstone is continuously cracked in "An Indirect Kiss". It may also interfere with the physical form itself. In "Bubbled", Eyeball's gemstone is cracked, resulting in her hand manifesting and unmanifesting several times. Shattering a Gemstone effectively kills the Gem (although it retains some magical properties, such as the Gem Shards in "Frybo" and "Secret Team", the Smoke Monster in "Together Breakfast" and all the gem shards they have encountered).

During Gem Fusion, all participants' gemstones shift colors to match each other. If the participants of the fusion are the same Gem type, the resulting fusion is just a larger version of the Gem.

Multiple Gems with the same gemstone may exist. Gemstone placement appears to vary from Gem to Gem, even if they are of the same gemstone. Most Gems of the same gemstone will have the same cut to their gemstone; however, some Gems such as the Diamonds, Pearl, Jasper and one of the Zoo Amethysts are exceptions to this. Gems of the same gemstone can have different summoned weapons.

Gemstones are generally locked in a specific location on the Gem's construct body, with even fusions maintaining the gemstones that compose them in the same anatomy. However, they can be oriented differently, as with Pink Diamond shifting her abdominally-located gemstone from a lateral to a dorso-ventral position as Rose Quartz.

Abilities
Gems can manifest things from within their gemstone, with the most important being their personal weapons and their physical bodies. Pearl, Garnet, and Peridot have the ability to store and withdraw objects (such as a roll of tape, temple key or an EMP device respectively) in their gemstone, but it is currently unknown if that ability is unique to them or if all Gems share it.

Gem bubbles have a stasis-like effect on gemstones; when "bubbled" by another Gem, the gemstone is prevented from regenerating/manifesting its physical body or weapon, essentially being imprisoned indefinitely until the bubble bursts.

Gemstones also find use in Gem technology. They are used as power sources for various objects such as Lapis' gemstone being used to power the magical mirror in "Mirror Gem". A corrupted gemstone also powered the mechanical apparatus in the Pyramid Temple in "Serious Steven".

Culture
As one would expect, the gemstone of each Gem is a core aspect of Gem culture. The Gems are not only named after their gemstone but are also recognized by other Gems by it. In "The Return", Jasper showed that she had known of Rose Quartz before the events of the episode and, due to Steven having inherited his mother's gemstone, she instantly thinks he is his mother shape-shifted into a weak, human boy which confuses and infuriates her. Individuality is a rare concept among Gems, as there is evidently little to differentiate Gems (aside from gemstone type) other than a serial number based on facet and cut.

It also seems to be customary for a Gem to keep their gemstones exposed, as all full uncorrupted Gems seen so far in the series have had theirs at least partially visible, to the point of altering the Homeworld uniform.

Trivia

 * Many known Gems are also real-world birthstones (main or alternatives). All months (except December) have had their main birthstone appear (bolded), while March, June, August, and December have alternate birthstones that have not appeared (italicized).
 * January:
 * Main: Garnet (Hessonite)
 * February:
 * Main: Amethyst (Others)
 * March:
 * Main: Aquamarine
 * Alternates: Bloodstone (Ronaldo)
 * Other: Jasper ("Skinny", Biggs, Others)
 * April:
 * Main: Diamond (Yellow, Blue, White, Pink)
 * May:
 * Main: Emerald
 * June:
 * Main: Pearl (Yellow, Blue)
 * Alternates: Alexandrite; Moonstone
 * July:
 * Main: Ruby (Others)
 * Alternates: Carnelian
 * August:
 * Main: Peridot ("Squaridot")
 * Alternates: Spinel
 * Other: Sardonyx; Agate (Holly Blue)
 * September:
 * Main: Sapphire (Padparadscha)
 * Alternates: Lapis Lazuli
 * October:
 * Main: Opal
 * Alternates: Tourmaline (Watermelon Tourmaline)
 * November:
 * Main: Topaz (Others)
 * Alternates: Citrine (Hessonite's, 67 Elite Citrines)
 * December:
 * Main: Turquoise
 * Alternates: Zircon (Defense, Prosecuting); Tanzanite
 * The real life 'meanings' of gemstones often correspond with some Gems' personalities.
 * Amethyst is a stone 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.
 * This is also shown with the other Amethysts shown, who are good-natured.
 * Pearl is a stone associated with purity. Pearl is mature, wise, and perfectionistic.
 * Garnet is a stone that protects from evil. Garnet is the most powerful of the Crystal Gems and often saves the day. She is also the current de facto leader of the Crystal Gems.
 * Rubies are also associated with protection, and most Rubies are guards, therefore fulfilling a role of protection.
 * Sapphires on the other hand, are associated with wisdom and mind healing. Sapphire is shown to be very mindful and intelligent.
 * Rose Quartz is a stone associated with love and joy. Steven is the most caring, and upbeat of the Gems, and Rose Quartz was very loving and had a joyful personality.
 * Lapis Lazuli is a stone associated with gods and power. She is a powerful Gem, nearly capable of defeating the Crystal Gems while controlling the Earth's entire ocean, in the midst of a cracked gem.
 * Peridot is a stone associated with the heart chakra, bringing balance and relieving stress. Peridot is shown to be level-headed and calculated, if not passionate about her work. She is also physically resilient.
 * Jasper is a stone associated with compassion and maternity, ironic considering Jasper's savage and aggressive temperament.
 * However, the gemstone is also associated with fire, so this could lead to Jasper's ill-temper.
 * Despite this the other Jaspers shown were good-natured much like the Amethysts, so Jasper might be an exception.
 * Zircons, in real life, are known for "repelling negative energy", somewhat fitting with the defending Zircon's lawyer occupation.
 * Although the gemstone being exposed is largely cultural, given that some corrupted Gems have had them internally, Steven once expressed discomfort as a baby when his baby onesie covered his gemstone.
 * Their gemstone placement seems to correspond to their personalities, actions, and how they think.
 * Amethyst's gemstone is near to the location of a human heart; she tends to be fun-loving and often does whatever she wants.
 * Bismuth's gemstone placement is quite similar, in the sense that she "follows her heart", and her emotions tend to be intense.
 * Pearl's gemstone is on her forehead; she tends to over-think things, has a perfectionist mindset, as well as being very intelligent.
 * Garnet's gemstones are in her hands; she tends not to talk often and respond through action instead.
 * Ruby's gemstone is located on her left hand; the left arm is connected to the heart, indicating her emotionalness.
 * Ruby's and Sapphire's gemstones are located on one of their hands - when they fuse into Garnet, they become symmetrical and complete each other.
 * Steven's gemstone is over his stomach; he tends to go with his gut instinct and is often hungry, and he is very loving and caring.
 * Rose Quartz herself is very caring and loving, and also has "hunger" for knowledge about Earth. Addiotionally, the navel connects to the umbilical cord, and indicates a maternal nature.
 * Lapis Lazuli's gemstone is placed over the center of her spine; she sprouts wings from her gemstone and has been shown to be very vulnerable emotionally.
 * Rebecca Sugar has stated that a gemstone placed on the back indicates vulnerability and a lack of control over "what's going on with you."
 * Peridot's gemstone is on her forehead; she is shown to be very intelligent (a trait she shares with Pearl) and dedicated to her mission.
 * Jasper's gemstone is placed over her nose; she is very self-centered and primal; it may also indicate being "hard-nosed", or determined and tough-minded.
 * Yellow and Blue Diamond have their gems on their chests, as they are considered "matriarchs" and have a high placement in the caste.
 * Additionally, Blue Diamond's placement may refer to her merciful side.
 * Yellow Diamond's placement may refer to her snobbish and stuck up personality.
 * Aquamarine's tear-shaped gemstone is under her eye; this false tear shows her deceitful nature disguised by her innocent appearance. This tear similarity is even jokingly and sarcastically referenced by Aquamarine in "Stuck Together".
 * In real life culture, a teardrop tattoo placed under the eye can often mean the wearer has killed someone or that they or a loved one has spent time in prison.
 * Topaz has her components' gemstones on her ears; she tends to obey the caste system and silently listen to Aquamarine's orders. In "Stuck Together", Topaz also intently listens to Lars and Steven's emotional conversation, causing her to open up and go against Aquamarine's orders.
 * The most common gemstone placement so far is the chest.
 * Although all classified as gemstones, other materials have been called such. Examples are Bismuth's gem, which is a metal, and the Glass Ghost's gem, which is a glass shard.