It is no secret that a hundred years ago, Icelanders weren't very modern. They were seen a bit uncultivated in the eyes of their contemporaries.
Sardonyx fits all of this to a T. Being formed out of deceit (even though Pearl didn't mean to hurt Garnet) is a staple of this sign.
Ruby is so like that oh my gosh. As seen in “Keystone Motel”, Ruby literally finds reasons to stay mad at Pearl, and is extremely attached to Sapphire.