Amethyst Birthstone Diamond And Square Ring 10K White Gold10k white gold diamond and square amethyst birthstone ring. The perfect February birthstone ring or mothers ring, this 10 karat white gold band has square amethyst and diamonds alternating half way around the ring. All birthstone rings and mother's rings are available in all birthstones, although the April (diamond) version may cost more. Looks great worn alone, or as a stackable ring.