Sardonyx towers in size; she is so tall that her hands are actually taller than Steven. She has vermilion skin and pale peach/apricot-colored hair, styled into a tetrahedron with very rounded corners. Her torso and legs greatly resemble Garnet's but have less mass. As a fusion, she has four arms and inherits the same locations of her gemstones as her fused cooperators. She wears a black, tailcoat tuxedo-leotard with a tangerine bow tie and a small, black star on her chest. This tailcoat comes from Pearl's sash ribbon. The underside of the tail of her tailcoat is the same shade of tangerine as her bow tie. She wears two-toned, mauve-gray leggings under crimson tips surrounding the top part of her feet. On her first set of arms, she wears white gloves, exposing her hands except a white covering on her middle fingers like Garnet's. On her second set of arms, located on her abdomen, she wears white gloves that closely resemble Sapphire's gloves that extend to her elbows.
She wears round shades that resemble those of Alexandrite's, but are slightly transparent and tinted with a dark reddish-ginger-like color, which reveals that Sardonyx has four eyes, two on the bottom which are large and round, and two on the top which are thin, small, and narrow. Her top pair of eyes is magenta, while the bottom pair is crimson-colored. Sardonyx appears to have a small gap between her front teeth and plump lips. Her face is rather triangular, and her feet have small, bright pink diamonds on them.
What's this? An encore performance? Just couldn't get enough of me, could you?
Sardonyx appears to have a combination of Garnet and Pearl's personalities, acting decidedly theatrical while also being able to keep up an intelligent conversation. Her demeanor is, for the most part, quite dramatic, as she prefers conversing with her peers in a matter similar to how a master of ceremonies would to her audience, which fits her formal dress appearance. She seems to be snarky and witty, often laughs at her own jokes, and remarks in a very pompous and comedic manner. She also seems to hold a very cheerful attitude towards almost anything as seen when she takes down the Communication Hub. She thinks very highly of herself and can be rather cocky at times, occasionally forgetting about others' feelings. Despite this, Sardonyx is completely capable of being serious when necessary; when Garnet and Pearl fuse to escape a crushing deathtrap, Sardonyx breaks out without hamming it up or sticking around any longer than necessary.
Sardonyx, like the rest of the Crystal Gems, has a positive opinion of Steven, as she interacted with Steven frequently, is happy to meet him, embraces him, and calls him a "sweet little charmer". Sardonyx has shown that she is quite fond of Steven, telling him jokes and performing tricks. She even compares herself to the more destructive Sugilite to impress him even more. While doing this, she shows off her anatomy manipulation, precision with her weapon, and names of the qualities about herself that counter Sugilite's.
Amethyst takes a disliking to Sardonyx because she compares her to her own fusion with Garnet, which is very volatile and dangerous. In addition to that, Sardonyx generally ignored Amethyst, by instead talking to and entertaining Steven. She also boasts her better qualities around Amethyst, making Amethyst feel responsible for the Sugilite incident seen in "Coach Steven". In Back to the Moon, Amethyst is seen on Sardonyx's shoulder while forming and hitting the Quintuple Ruby Fusion. Smoky Quartz shows no dislike toward Sardonyx, showing that Amethyst may have gotten over her dislike.
Sardonyx first meets Smoky Quartz after Steven and Amethyst fuse to allow her to meet them. Sardonyx takes them into her room in the temple, which is a late night talk show where she interviews Smoky on who they really are, forcing them to play games to try to find out what their special abilities are. This shows that, while she is interested in Smoky, she is very self-centered in her treatment of them, which causes Sardonyx to defuse when Pearl and Garnet realize how they were acting.
Although never meeting Jasper before, she mentions her on her own show, Sardonyx Tonight, that she "Always runs around Kindergartens causing trouble", joking about her that she should go back to Preschool. This implies that Sardonyx doesn't have a kind heart for Jasper, calling her a trouble-maker.
Sardonyx possesses standard Gem abilities and traits.
When in battle, Sardonyx is both graceful and accurate, but also powerful and rather playful, messing with her opponents or audience with tricks and jokes. Despite this, she is usually quite tactical, thinking things up before acting, to make things perfect.
War Hammer Proficiency: Sardonyx can combine Garnet's gauntlets and put them on the end of Pearl's spear facing inwards to make a hammer. Her use of the war hammer is methodical in approach. She uses precise, keen strikes on specific areas to inflict the most damage while also reducing the amount of effort needed to take down a structure or opponent. She describes herself, and therefore her style of combat, as "specific, intelligent, accurate, faultless, elegant, controlled, surgical, graceful, and powerful!" (But yes, occasionally, she is known to smash).
Drill Transformation: By rapidly spinning the top part of her hammer, Sardonyx can transform her weapon into a drill that could easily break through the walls of the Ancient Gem Colony Ship that Garnet's gauntlets and Pearl's spear alone could not even dent.
Anatomy Manipulation: Sardonyx has been shown to have "doll joints", body segments that have full rotation independent of the rest of her body. During her initial attack on the rebuilt Communication Hub she was seen to spin her upper body above the sash while talking to Steven, her sash arms are seen moving separately from the rest of her body throughout the scene, and during the final strike her whole body is spinning on one joint while her feet remain pointed up and immobile. In "Back to the Moon", it's shown that not only can her waist spin in this fashion, but also her neck. She is able to swing her body around without taking her eyes off the Ruby fusion. In "Know Your Fusion", Sardonyx was able to move through the cramped space of the house and through the Temple Gate with minimal difficulty.
Enhanced Calculations: Sardonyx can knock out the correct pillars to make sure the hub did not collapse until she wanted it to. She may have inherited this from Garnet, who mentions her ability to sense structural integrity in the episode "Cheeseburger Backpack" and her future vision, she could see if she knocked down the wrong pillar, it would all collapse. She may have also inherited this from Pearl, who is precise and accurate.
As with Garnet, it is currently unknown whether this is a Gem ability/power that is inherited from Ruby/Sapphire unique to both Garnet and Sardonyx or a simple observation.
Due to being a Pearl fusion it is unknown if Sardonyx is acrobatic. A hint of her having this skill is when Amethyst is singing "Tower of Mistakes" Sardonyx is shown doing back flips while destroying the tower.
Sardonyx was announced in a Crewniverse Tumblr post.
She is revealed in a clip shown at the official Adventure Time/Steven Universe panel at San Diego Comic-Con 2015.
Sardonyx's outfit unusually features both diamonds and stars, with pink diamonds on her shoes, and a black star on her chest. Whether the diamonds are meaningful or not is unknown.
The pink diamond design on her shoes also appears on Pearl's outfit in "Space Race", though on the middle of her torso.
Sardonyx asks Steven if she is "worth the wait", referencing how long she took to appear in the show.
According to Matt Burnett, Garnet and Pearl fused into Sardonyx with a hand-based dance in "Friend Ship". This supports the fact that fusion dances may vary depending on the situation, the time they have, and how the Gems are feeling.
Her top set of eyes function as eyebrows, similar to Malachite.
Sardonyx can be formed if Ruby and Sapphire are not Garnet. 
Sardonyx's eyes occasionally glow a brighter orange when she is talking to Steven, telling jokes, or when summoning her weapon. It is unknown if this is from lighting, a mistake in animation, or if they naturally brighten when she gets excited.
According to Joe Johnston, he added a gap between Sardonyx's teeth because he also has a slight gap in his.
Her war hammer is made with the gauntlets facing inward, yet the design has them facing outward.
According to Aivi Tran and Surasshu, Sardonyx's theme song shows the influence of Garnet and Pearl's respective music on each other (and as such, their personalities as well.) 
Garnet’s presence has influenced Pearl’s piano to become confident and bombastic, while Pearl’s presence has influenced Garnet’s bass and synth sounds to become theatrical and jazzy.
Rebecca Sugar hopes for Sardonyx to have a musical number in the show. 
Her comical personality, coupled by her gloating of her own qualities around Amethyst, might be inspired by the word "sardonic", meaning "disdainfully or ironically humorous".
As shown in "Know Your Fusion", she shows evidence of being aware of the 4th wall, referencing directly Cartoon Network, merchandising being necessary for her show to be on air, and Sugilite's voice actress being paid for her voice being featured despite not actually being in the episode nor recording any new lines.
Sardonyx is the only fusion so far that has been seen intentionally de-fusing more than once.
Fans have noted there are many similarities between Sardonyx, and the PlatinumGames character Bayonetta. Both pose dramatically in front of the moon before performing an attack, both are classy and self-confident, graceful yet incredibly strong, and both have a fondness for children (Bayonetta with Cereza & Loki, Sardonyx with Steven). They both taunt before finishing off their targets, and both have the same snarky means of addressing bystanders during battle.
Coincidentally, the way Sardonyx uses her war hammer is identical to how Bayonetta uses her Hammer weapon, the Takemikazuchi, from Bayonetta 2.
Her outfit is similar to a traditional mid-20th-century stage magician, show hostess, and Broadway dancer costumes.
The way she appears (pulling and opening a curtain that explodes into dragonflies) also plays into her stage magician theme.
Also when she summons Pearl's spear and Garnet's gauntlets to create the war hammer, her actions resemble that of a performer.
It fits in perfectly with the way she talks, how she acts, and her personality, evocative of that type of entertainer.
Her showman-like personality is in many ways similar to Ruby Rhod, Chris Tucker's character in The Fifth Element.
Based on her quote "But yes, occasionally I am known to smash", fans jokingly asked Matt Burnett which character Sardonyx uses the most, or "mains", in Super Smash Bros. was, to which he replied "King Dedede", a character in the game who also uses a giant hammer.
Sardonyx's hairstyle resembles an onigiri rice ball. This is referenced in a sketch by Hilary Florido.
Sardonyx's defusion resembles smoke bombs used by magicians when they disappear.
Sardonyx is a type of microcrystalline quartz, specifically onyx, banded with browns, reds, or yellows instead of black.
Onyx and sardonyx are types of agate, which is the gemstone of those with the sign Gemini.
Sardonyx is closely related to onyx, sard, and carnelian. Banded agate with more blacks or grays is generally classified as onyx, whereas banded agate with more browns, reds, yellows, or oranges is generally classified as sardonyx. Solid red or reddish-brown agate is generally classified as sard or carnelian, depending on the shade.
Sardonyx, as with many other gemstones, can also appear as other colors, such as green and purple.
The name sardonyx itself is an amalgamation of its composition, the two minerals ‘sard’ and ‘onyx’.
Sardonyx is said to enhance willpower, vitality, confidence, stamina, integrity and optimism as well as bring happiness and stability to marriages.
Sardonyx's personality and behavior, as well as Pearl's description of being fused with Garnet, seems to match perfectly with the attributes of her gemstone.
Sardonyx is an alternate birthstone of August, whereas the official birthstone is peridot.