Black agate. According to Cartier official website's inquiry, Cartier 18k white gold jade pendant necklace with black agate ring is novel and unique in style, exquisite in workmanship, beautiful and elegant to wear, showing nobility. Cartier is a famous French jewelry and watch manufacturer. It was founded by Louis Fran? ois Cartier in 1847 in Ruemontogueil 3 1, and is regarded as one of the most famous jewelry manufacturers in the world.