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's 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) of a Gem is severely damaged or injured, it retreats to its gemstone to regenerate and project a new form. All Cristal 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". Destroying a Gemstone causes the gem to be unable regain its intended form (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've encountered).

During Gem Fusion, all participants' gems 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. All Gems of the same gemstone will have the same cut to their gemstone; the Diamonds and Pearls are exceptions to this.

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 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) than a serial number based on facet and cut.