"Serendibite" is a member of an unnamed Gem rebel group.
Serendibite is a fairly young gem who served under Blue Diamond's court. She was a lower-class Gem.
She has since joined a Gem Rebel group lead by Grandidierite.
Real world serendibites are considered to be one of the rarest gemstones in the world, which is quite the opposite in this universe. Depending on the planet, rarity can range from uncommon to semi-rare.
Serendibite Gems typically display dark colours unless exposed to polarized light, in which the true colours of their gemstone and even clothing become more prominent.