Amethyst is a stunted Era 2 Quartz Gem that had been born on Earth 5,500 years ago. She came out in the Prime Kindergarten, but as of current, resides with the Crystal Gems. Amethyst avoided the Corruption Light because she was in the ground at the time, and therefore inactive.
As far as most Quartz Gems go, Amethyst is significantly smaller than most of them, about half the size of a standard Quartz Gem would be, mainly due to her being in the ground longer than her fellow Amethysts. Her skin is purple in coloration, with her long hair reaching down her back, almost touching the ground, and in a lighter hue of lavender.
In her first appearance, Amethyst wore a oversized, black tank top, with leggings colored mauve with two black stars engraved on the knees. On the lower part of her tank top, there are two small slits on either side, and she wore white boots. As of "Prey for the Beasts", Amethyst's color palette remains the same but has been swapped: her tank top is now off-white with a jagged hem, and she now has black leggings with dark mauve stars on them. Her ankle boots are now the same dark mauve hue.
Amethyst is a more outgoing, carefree, and tomboyish individual amongst the Crystal Gems. She is a person who's full of laughter, and someone one would consider as a fun-loving "big sister". She acts out of impulse, and finds a bit of enjoyment in the more action-orientated moments, though enjoys relaxation as much as the next guy, if not a bit more so. She is the most comfortable around Humans, and much like Emerald is more accustom to their lifestyle and ways than some of the other Gems. While she doesn't need to in order to live, she does enjoy eating various objects, edible or not, and sleeping as well. In accordance to her room, she is shown to be somewhat of a hoarder, a trait she had from an early age, and considers much of it as decoration if not much else.
She secretly felt guilty for being the last made in the Prime Kindergarten, thinking herself as something bad due to what the Kindergarten was doing to the planet. This is explored in the My Little Universe: Mindwarp special, her having to fight a larger, yet more brutish and cruel version of herself and who she envisioned herself as due to this guilt. This is touched on further during Season 5 when she and Steven had to fight eachother, though to prove that she was the worst one to Steven, just to have it confirmed by Yellow Diamond, making her feel worse about herself. This is seemingly pushed over though after her talk with Acid and Cyanide for the first time, albeit accidentally.
Aside from her guilt to her existence, is her desire to be special, and to be as good if not better than how she was originally intended to be. As such, she showed a deep need to be with others, and not wanting to be alone and useless to others' eyes. This is touched on numerous times throughout the series, such as when she tried to alter the past in the Season 3 finale so her friends would still be with her, and further explored throughout Season 5 and the situation regarding her, Steven Universe, and Connie Maheswaran in terms of fusion.
Out of all the Crystal Gems, or in other Gems presented for that matter, Amethyst has been known to shapeshift the most, and shapeshift the most easily. This is due to her extra time in the Earth, and as such she is able to shapeshift into all sorts of objects and animals with little strain or stress on herself.