Gemstones

"Gemstones" are the signature part of any Gem. They appear as gemstones in various cuts, shapes, and colors located on any part of a Gem's physical form. It also shares the gems name.

Description
The gemstones are, in essence, the Gem being itself. According to Pearl in "What Are Gems?", their physical bodies are merely a projection with mass from within the gemstone, much like their weapons and outfits. Garnet states in "Steven the Sword Fighter" that if the physical body (that is just an illusion) of a Gem is damaged or injured, it retreats to its gemstone to regenerate and project a new body form. All Crystal Gems besides Steven were 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 was unable to summon her water wings while her gemstone was 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 was continuously cracked in "An Indirect Kiss". Destroying a Gemstone effectively kills the Gem (although it  retains some magicical properties, such as the Gem Shards in "Frybo" and "Secret Team" and the Smoke Monster in "Together Breakfast").

During Gem Fusion, all participants' gems shift colors to match each other.

So far, every gemstone has been unique (excluding fusions) except for the matching gems of the Heaven Beetle and Earth Beetle.

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

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

Gemstones also find a 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 gemstone also powered the mechanical apparatus in the Pyramid Temple in "Serious Steven", and Peridot used her gemstone to summon her Escape Pod. It should be noted that the ethical implications of using gemstones in this manner has yet to been fully addressed in the series; however Lapis' case was forced servitude as she desired escape as well as referred to her existence within the mirror as "imprisonment". The experience also severely traumatized her to such a degree that it left a deep psychological scar that affected her personality.

Culture
As one would expect, the gemstone of each individual 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 prior to the events of the episode and, due to Steven having inherited his mother's gemstone, she instantly thought he was his mother shape-shifted into a weak, human boy which confused and infuriated her.

Trivia

 * List of known Crystal Gems and their ties to birthstones (by month).
 * Garnet - January
 * Steven/Rose Quartz - January (Alternative)
 * Amethyst - February
 * Pearl - June
 * Alexandrite - June (Alternative)
 * Ruby - July
 * Sardonyx - August (Alternative)
 * Sapphire - September
 * Opal - October
 * The gems themselves correspond with the 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.
 * 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 leader of the Crystal Gems.
 * 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, 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.
 * 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.
 * Their gemstone placement seems to correspond to their personalities, actions, and how they think.
 * Amethyst's gem is near to the placement of a human heart; she tends to be fun-loving, and often does whatever she wants.
 * Pearl's gem is on her forehead; she tends to over-think things, has a perfectionist mindset, as well as being very intelligent.
 * 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, and like his mother he is very loving and caring.
 * Lapis Lazuli's gem is placed over the center of her spine; she sprouts wings from her gem and has been shown to be very vulnerable emotionally.
 * Peridot's gem is on her forehead; she is shown to be very intelligent (a trait she shares with Pearl) and dedicated to her mission.
 * Jasper's gem is placed over her nose; she is very self-centered and primal.