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. 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.
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, although she also seems to be considerate of other people as well as her surroundings. Despite this, Sardonyx is completely capable of being serious when necessary.
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.