Obsidian

"Garnet: We need to be bigger. Steven: Then let's fuse, all four of us! It's the only way we're gonna get up to her head! Amethyst: I'm in! Pearl: I'm in, too. Garnet: Then it's decided. It's time to form Obsidian."

Obsidian is the fusion between Garnet (by extension, Ruby and Sapphire), Amethyst, Pearl, and Steven Universe. They made their debut in "Change Your Mind".

Appearance
"They're huge!" Obsidian is a massive fusion with eight arms, glowing orange eyes, gray skin, and charcoal-black hair that reaches their knees. The skin and hair have bright orange markings. They also flaunt a mask, which opens up to a third mouth. The third mouth is filled with lava that Obsidian uses to forge their sword. Their appearance greatly resembles that of the Crystal Temple's statue figure.

Personality
Obsidian does not appear to have a distinct personality, as they are shown to be always silent. The only noises they make are loud roars.

"Change Your Mind"
In an effort to ascend the complete Diamond Mech, Steven suggests that the four core members of the Crystal Gems should fuse. The other Gems agree, and Garnet declares that it is time to form Obsidian. Garnet, Steven, Pearl, and Amethyst soon perform a fusion dance before coming together to form Obsidian. As Obsidian lets out a roar, the Diamond Mech soon returns. Obsidian begins to climb up the mech while Connie, Bismuth, Peridot and Lapis distract the upper part of the mech.

Abilities
Obsidian possesses standard Gem abilities and traits. As the fusion of all four core members of the Crystal Gems, Obsidian is the strongest fusion so far in the series, stronger than both Malachite and Alexandrite.

Skillset

 * Greatsword Proficiency: Obsidian forms the hilt of their sword by combining Pearl's spear, Steven's shield, Garnet's gauntlet, and Amethyst's whip; then, Obsidian smelts the blade of the sword by opening up a third mouth full of lava and sticking the hilt into their mouth. The red-hot sword that they pull out of their mouth is then capable of slicing through nearly anything, as it destroys Yellow Diamond and Blue Diamond's ships with seeming ease.

Unique Abilities

 * Pyrokinesis: When Obsidian was eventually formed, their eyes (and in their mask) ignited fire.
 * Lava Generation: Obsidian's third mouth acts as a forge and is filled with lava, which they use to craft their greatsword.

Trivia

 * The fact that Obsidian has the ability to create/hold lava within their body may be a reference to the fact that, in real life, obsidian is a type of volcanic glass formed when felsic lava cools.
 * One of the uses of obsidian in human history is as material for cutting and piercing tools. This is referenced through Obsidian's weapon of choice in the form of a sword, its extreme sharpness mirroring the real-life obsidian's own.
 * Obsidian bears a resemblance to Te Kā from Moana, mainly because of their volcanic-like appearance and abilities.
 * This Fusion also bears some resemblance to the final monster created by X.A.N.A. in Code Lyoko, the Kolossus. Both are titanic molten lava creatures with large swords as their primary weapon.
 * The name of Obsidian was first seen in the Steven Universe: Art & Origins book.
 * Obsidian's sword strongly resembles both the Flameblade and Great Frostblade from The Legend of Zelda: Breath of the Wild. The sword's fiery properties and color resemble the Flameblade, the sword blade's outline more resembles the Great Frostblade, and all three blades have geometric patterns on the blade surface.
 * In Change Your Mind, before fusing, Garnet mentions Obsidian's name, implying that she, Amethyst, Pearl and Pink Diamond/Rose Quartz had fused together before.