Gem Fusions are a subspecies of Gem from the animated series Steven Universe. The body of a gem is made of solidified light with specific wavelengths, frequencies, and colors. A gem can synchronize their physical form with that of another gem to fuse with them. This is very easy for gems of the same type since their wavelengths, frequencies, and colors are already the same. All they need to do is make physical contact with each other and share the same intentions. With gems of different types, they must take the time to synchronize themselves physically, mentally, and emotionally and must maintain deep concentration and single-minded focus on their intentions in order to create and maintain the fusion. This is most easily accomplished by the two gems dancing with each other but this may also occur between two gems in moments of physical and emotional intimacy.

When two or more gems fuse together, the resulting gem is much larger and stronger than the individual components and combines personality traits and design elements from the components. Gems of the same type will result in a larger version of that same gem. However, Gems of different types will result in a completely new type of gem with monstrous or chimeric physical properties and palettes including extra limbs and organs.

Gem fusions also combine the summonable weapons of the individual gem components to create new weapons. For example: Rose Quartz's Weapon is a shield and Pearl's weapon is a spear, so combined their weapon becomes a parasol which they can use to fly through the air like Mary Poppins or as a cross between a sword and a shield not unlike Parasoul from Skullgirls or The Penguin from Batman.

Gem fusions are not limited to two people, as it is possible for many different gems to fuse. This is especially obvious give that Garnet herself is a fusion and frequently fuses with other characters throughout the series, essentially creating a 3-way fusion. However, the more gems integrated into a fusion, the less stable the fusion becomes as the component personalities may vie for dominance and become more distinct, destabilizing and even breaking the synchronization. Gems with inherently incompatible personalities may also struggle to successfully fuse to begin with such as Amethyst and Pearl. Currently, the most complex fusion is Fluorite who is composed of 6 different gems. Like Garnet, she is a perma-fusion and while she is very stable, she thinks, moves, and speaks very slowly and by far has the least humanoid appearance for any fusion, taking the form of a giant caterpillar. According to Garnet, when you fuse:

...you are not two people. And you are not one person. You are an experience.

According to Garnet, fusion is also an extremely interment experience that is best meant as the ultimate expression of love. Gem's who choose to stay fused almost constantly are known as "perma-fusions" and the act of being fused to a gem this way is widely seen as the equivalent of being in a sexual relationship with a person.

Steven Quartz DeMayo Universe is the first and only Human/Gem Hybrid and uniquely has the ability to fuse with both Gems and Humans. When fusing with Gems, Steven's Fusions have a distinctly more masculine or boyish appearance compared to the versions of fusions achieved by his mother. When fusing with other humans, the resulting fusion has the physical age of Steven plus whomever he is fusing with. For example, the fusion of Steven (16) and his friend Connie (15) results in Stevonnie who is 31 years old. This also combines the sex and genders of the components with Stevonnie being gender non-binary/intersex and Steg, the fusion of Steven and his father Greg being completely male and well endowed. The only other Gem with anything remotely like this ability is Topaz who can fuse their bodies around organic components, entrapping them.

Following the Gem War, the Order of the Diamonds attempted to create a massive, planet-sized, geo-weapon made from millions of broken gem shards from casualties of the Gem War. This abomination was created by artificially fusing gems by attaching the gem shards and incubating it within the earth's crust. This entity became known as "The Cluster".

The following are a list of fusions that appear in the series:

• Single
• Stevonnie (hybrid fusion: 1/4 Gem, 3/4 Human)
• Steven Universe (hybrid: 1/2 Gem, 1/2 Human)
• Connie Maheswaran (Human)
• Steg (hybrid fusion: 1/4 Gem, 3/4 Human)
• Steven Universe
• Greg Universe (Human)
• Double
• Garnet
• Ruby
• Sapphire
• Opal
• Amethyst
• Pearl
• Rainbow Quartz
• Rose Quartz/Pink Diamond
• Pearl
• Malachite
• Lapis Lazuli
• Jasper
• Smoky Quartz (hybrid fusion: 3/4 Gem, 1/4 Human)
• Amethyst
• Steven Universe
• Zebra Jasper (semi-corrupted fusion)
• Jasper
• Ocean Jasper (corrupted)
• Topaz
• Topaz (left ear)
• Topaz (right ear)
• Rhodonite
• Unknown Ruby
• Unknown Pearl
• Lemon Jade
• Green-Yellow Jade
• Blue-Green Jade
• Rainbow Quartz 2.0 (hybrid fusion: 3/4 Gem, 1/4 Human)
• Pearl
• Steven Universe
• Mega Pearl
• Pearl
• Pink Pearl
• Bluebird Azurite
• Aquamarine
• "Eyeball" Ruby
• Topaz Chamber Guard
• Topaz (left shoulder)
• Topaz (right shoulder)
• Crazy Lace Agate
• Two unknown Quartz components
• Watermelon Tourmaline
• Two unknown Tourmaline components

• Triple
• Sugilite
• Ruby
• Sapphire
• Amethyst
• Sardonyx
• Ruby
• Sapphire
• Pearl
• Sunstone (hybrid fusion: 5/6 Gem, 1/6 Human)
• Ruby
• Sapphire
• Steven Universe
• Ruby (triple fusion)
• Ruby (Crystal Gem)
• Ruby (right shoulder gem)
• Ruby (right back-hand gem)
• Prism Fusion

• Quadruple
• Alexandrite
• Ruby
• Sapphire
• Pearl
• Amethyst
• Hand Cluster (forced fusion)
• 4 Former Crystal Gems

• Quintuple
• "Giant Ruby"
• "Doc"
• "Eyeball"
• "Army"
• "Navy"
• "Leggy"
• Obsidian (Rose Quartz Fusion)
• Ruby
• Sapphire
• Amethyst
• Pearl
• Rose Quartz
• Obsidian (Steven Fusion) (hybrid fusion: 9/10 Gem, 1/10 Human)
• Ruby
• Sapphire
• Amethyst
• Pearl
• Steven Universe

• Sextuple
• Fluorite
• Six unknown Gems

• Millions
• The Cluster (forced fusion)