In Greek mythology, Poseidon was the supreme ruler of the seas. When angry, he could cause violent natural disasters like earthquakes and storms, but if he was content, he'd protect sailors at sea and guide ships to safety. Aquamarine would have been his totem gem; since antiquity the blue stone has been associated with water and the ocean. Superstitious seagoers often carried it as an amulet to prevent drowning and stave off seasickness.