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."

Obsidian is the fusion of Garnet (and by extension Ruby and Sapphire), Amethyst, Pearl, and Steven Universe. They formed for the very first time in their debut episode, "Change Your Mind".

Appearance
"They're huuuuge!" Obsidian is a massive fusion with eight arms, glowing orange eyes, gray skin, and slightly curly charcoal-black hair that reaches their knees. The skin and hair have bright orange markings, which appear to be lava. They have plump lips, and their knee caps are diamond shaped. Steven's clothes can be seen on one of Obsidian's hands, a reference to where Steven's laundry is hanged in the Crystal Temple.

They also flaunt a mask (that features a nose similar to that of Pearl and plump lips similar to Garnet and Amethyst's), 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.

All the components' gemstones appear solid black.

Personality
Obsidian does not speak, only making vocalizations such as growls and roars. They have a ferocious side, as demonstrated when they roared into the eye of the Diamond Mech. However, she does retain her love of her friends as she protected Connie from falling.

"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 tumultuous roar and the other Crystal Gems express their awe in seeing Obsidian, the Diamond Mech soon returns. Obsidian begins to climb up the mech briskly while Connie, Bismuth, Peridot, and Lapis distract the upper part of the mech.

After ascending most of the mech, Obsidian begins to summon their weapon, a gigantic greatsword. Doing so leads to the attack of the Diamond Mech, managing to slice the wrist of the mech's right arm, and in half for the left arm. It then proceeds to immediately thrust the sword into the mech's chest, and Obsidian then leaps onto the mech's face, roaring into its eye. Thrown off balance, the Diamond Mech proceeds to tumble throughout the Homeworld city, ramming its face into a pillar and unfusing Obsidian.

Abilities
Obsidian possesses standard Gem abilities and traits. As the fusion of all four (five, counting Ruby and Sapphire separately) core members of the Crystal Gems, Obsidian is presumably the strongest fusion so far in the series, possibly 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. The sword has a series of runes that run up the length of the blade, though the meaning of these runes are unknown.

Unique Abilities

 * Enhanced Grip: Obsidian can scale any surface, such as the Diamond Mech, easily using just their bare hands, and without the need of extra tools such as Sunstone's suction cups.
 * Pyrokinesis: When Obsidian was eventually formed, their eyes produced a fiery aura.
 * Lava Generation: Obsidian's third mouth acts as a forge and is filled with lava, which they use to craft their greatsword.
 * Future Vision: Obsidian possesses Sapphire's Future Vision.

Connie Maheswaran
"I'll let you take this one." Connie, along with the other Crystal Gems, was in awe when first witnessing Obsidian's appearance.

Just like her components, Obsidian cares about Connie's well-being. This is demonstrated when they save Connie from falling to her death. After being caught, Connie draws her sword in an attempt to take on the mech herself, but decides to let Obsidian handle it after they summon their own sword.

Trivia

 * The fact that Obsidian has the ability to create/hold lava within her 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 blades were the primary material components of Aztec weapons like the Macuahuitl.
 * The name of Obsidian was first seen in the Steven Universe: Art & Origins book, as the beta name for the Crystal Temple (there called the "Obsidian Temple").
 * Obsidian's sword strongly resembles both the Flameblade and the Great Flameblade from The Legend of Zelda: Breath of the Wild. The sword's fiery properties and color resemble the Flameblade and Great Flameblade while the sword blade's outline more resembles the Great Frostblade, and all three blades have geometric patterns on the blade surface.
 * Obsidian was formed before with Rose. Rose's Obsidian fusion is the basis of the statue that makes up the Crystal Temple, and was the fusion that led William Dewey to the beach where he established Beach City.
 * According to Joe Johnston's Tumblr, Obsidian uses both she/her and they/them pronouns.
 * Obsidian bears a resemblance to the Hindu war goddess Kali, who is commonly depicted as an ash or blue-skinned warrior woman with many arms and long black hair. However, this is unconfirmed to be the design inspiration of Obsidian.
 * Obsidian is the first 5 cross-gem fusion seen in the show.
 * Due to having all of the main Crystal Gems weapons, Obsidian should also be able to summon Sugilite's flail, Opal's bow, Sardonyx's war hammer, Rainbow Quartz 2.0's parasol, Sunstone's suction cups, Smoky Quartz's yo-yo, and perhaps the utility of any other possible fusion by Ruby, Sapphire, Amethyst, Pearl, and Steven.
 * It is noted by Rebecca Sugar that Obsidian has Garnet's Future Vision ability, though it is useless as Obsidian's immense power makes her future vision a "cacophony of noise", much like all of her abilities.
 * She is one of the two cross-gem fusions seen so far to have a singular Gem counterpart, next to Garnet with Hessonite. Hers is Snowflake Obsidian, another Crystal Gem.
 * Obsidian's formation to combat White Diamond may be a reference to the fact that while diamonds are a crystalline mineral, obsidian is not an actual mineral (it is, instead, mineral-like glass) and lacks a crystal structure entirely, as the lava that forms it cools too quickly for crystals to form. This is ironic given that she is the fusion of all the core Crystal Gems.
 * Additionally, her coloration could be a reference to colors in terms of light versus pigmentation. In light, white is the presence of all colors and the absence of colors in pigmentation. Black is the absence of light, but the presence of multiple or all colors in pigmentation.
 * Obsidian is Garnet's only known fusion who doesn't wear some variation of her visor.
 * Despite the inclusion of Pearl and Steven, Obsidian has fewer eyes than Sugilite (four as opposed to Sugilite's five).

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