Chalcedony is considered to be the May birthstone, and it is also considered to be the zodiac stone for Gemini. Also, bloodstone, one of the chalcedony varieties, is a March birthstone. Are you celebrating your wedding anniversary soon? If yes, then the chalcedony varieties have got you covered. Among the various chalcedony varieties, onyx is considered to be the traditional 7th-anniversary gemstone, carnelian is considered to be the 17th, and chrysoprase is considered to be the 18th-anniversary gemstone.