This article contains information that is considered canon in the "Steven Universe" universe.
"Gemstones" are the signature part of any Gem. They appear as literal gemstones in various cuts, shapes, and colors located on a part of a Gem's physical form.
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 severely 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 magical 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.
Multiple Gems with the same gemstone may exist, Peridot has revealed that there are multiple Gems with the same gemstone type. Gemstone placement appears to change from gem to gem, even if they are of the same gemstone.
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 bursts.
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.
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.
|Rose Quartz's gemstone, along with Steven's, is located on their navels. It has a pentagonal facet.||"Gem Glow"|
|Pearl's gemstone is located in the center of her forehead. The gemstone is smooth with no facets. As seen when she is regenerating in "Steven the Sword Fighter", it has a silver ring around the edge which is usually hidden inside Pearl's forehead.|
|Amethyst's gemstone is located on her chest. It has a hexagonal facet. As seen in "Reformed", it has a silver ring around the edge which is normally hidden inside Amethyst's body unless she's regenerating.|
|Ruby's gemstone is located on the palm of her left hand. It has a square facet. As seen in "The Return" and the flashback in "Jail Break", it has a light red ring around the edge (normally hidden inside her hand), and is flatter on the back.||"Jail Break"|
|Sapphire's gemstone is located on the palm of her right hand. It has a triangular facet. As seen in "The Return" and the flashback in "Jail Break", it has a light blue ring around the edge (normally hidden inside her hand), and is flatter on the back.|
|Lapis Lazuli's gemstone is located in between her shoulder blades. Her gemstone is smooth with no facets.||"Mirror Gem"|
|Peridot's gemstone is located in the center of her forehead. Her gemstone has facets around the edge.||"Warp Tour"|
|Jasper's gemstone is located where her nose would be, in the center of her face. Her gem is cut with a point that is the conversion of the other three facets||"The Return"|
|Yellow Diamond's gemstone is located on her chest. It has a diamond shaped facet.||"Message Received"|
|Blue Diamond's gemstone is located on her chest. It has a diamond shaped facet.||"The Answer"|
|The Centipeetle Mother's gemstone is located in the Oropharynx (back of the throat). It is a sphere with a "pupil" in the center.||"Gem Glow"|
|The Lighthouse Gem Monster's gemstone does not have a known physical location. It has triangular facets and a circular center facet.||"Horror Club"|
|The Ice Monster's gemstone is in the center of its head (tridimensional center). It appears to have 6 facets and an oblong-shaped center facet, despite the "crack" on the 6th facet.||"Monster Buddies"|
|The Invisible Gem Monster's gemstone is located on the middle of its chest. It has two facets tenting at the middle of the front part of the gem. The back of the gem is cone-shaped.||"Island Adventure"|
|The Worm Monster's gemstone is located in the Oropharynx. It has 7 facets that meet at the center.||"Bubble Buddies"|
|The Gem Cave Creature's gemstone is located somewhere within its central disc. It is shaped like an icosahedron, with spikes at each point of a triangle-shaped facet.||"Arcade Mania"|
|The Water Bear's gemstone is located in the mouth. It is shaped like a cone, and has a single circular facet.||"Love Letters"|
|The Slinker's gemstone is located inside the junction between the body and the head. It is a cabochon oblong.||"Reformed"|
|The Heaven Beetle and Earth Beetle's gemstones are located on their backs. They are both shaped like diamonds, and have four facets with a diamond-shaped center facet.||"Giant Woman"|
|The Crab Gem Monster's gemstone is located at its iris. It appears green due to the color pallete.||"Rising Tides, Crashing Skies"|
|The Desert Glass gemstone does not have a known physical location; it resembles desert glass. It is circular, and has many facets in abstract shapes.||"Steven's Lion"|
|The Pyramid Temple's gemstone is located in the center of the Temple within the Obelisk. It is shaped like a equadilateral diamond, and has four facets with a smaller sized diamond-shaped center facet.||"Serious Steven"|
|Big Bird's shards do not have a known location but are each the gem of a much smaller bird.||"Giant Woman"|
|Amethyst's cracked gem.||"An Indirect Kiss"|
|Lapis Lazuli's cracked gem.||"Mirror Gem"|
|Steven's gemstone on Stevonnie.||"Alone Together"|
|Pearl's gemstone on Rainbow Quartz.||"We Need to Talk"|
|Rose's gemstone on Rainbow Quartz.|
|Ruby's gemstone on Garnet.||"Gem Glow"|
|Sapphire's gemstone on Garnet.|
|Pearl's gemstone on Opal.||"Giant Woman"|
|Amethyst's gemstone on Opal.|
|Amethyst's gemstone on Sugilite.||"Coach Steven"|
|Ruby's gemstone on Sugilite.|
|Sapphire's gemstone on Sugilite.|
|Pearl's gemstone on Sardonyx.||"Cry for Help"|
|Ruby's gemstone on Sardonyx.|
|Sapphire's gemstone on Sardonyx.|
|Pearl's gemstone on Alexandrite.||"Fusion Cuisine"|
|Amethyst's gemstone on Alexandrite.|
|Ruby's gemstone on Alexandrite.|
|Sapphire's gemstone on Alexandrite.|
|Lapis' gemstone on Malachite.||"Jail Break"|
|Jasper's gemstone on Malachite.|
Forced Fusion Gemstones
|The Hand Cluster's gemstone is a mix of four different gems joined into a single gemstone. It's located on the center of its "palm".|
|A smaller Cluster taking the form of two forearms has two Gem Shards fused together into a V-shape, with smaller pieces stuck around the joint. One is blue, the other is purple. The shards are internal.|
|The gemstone of the six-armed Cluster Gem - three shards joined at one point, one teal, another red, and the other gray.|
|The gemstone lumpy Cluster Gem. A squarish purple shard with small yellow and blue sards attached.|
|The gemstone of the Cluster with a Light Green head that has one eye and mouth on its face, a light blue body and left hand, a big green foot as it's right hand, and purple lower body.|
|The gemstone of the giant white hand cluster|
|The gemstone of the Blue Hand with seven fingers and a foot Cluster.|
|The gemstone of the cluster with a round violet body, three blue limbs, and an eye.|
|The gemstone of the small cluster with a blue hand and red leg.|
|The gemstone of the cluster with a big blue hand, and pink foot.|
- List of known Gems and their ties to birthstones (by month).
- Garnet - January
- Steven/Rose Quartz - January (Alternative) 
- Amethyst - February
- Yellow Diamond/Blue Diamond - April
- Pearl - June
- Alexandrite - June (Alternative)
- Ruby - July
- Peridot - August
- Sardonyx - August (Alternative)
- Sapphire - September
- Lapis Lazuli - September (Alternative)
- 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.