Most of the chalcedony stones are found in colors of blue, gray, or white. The blue chalcedony variety, in particular, is found to be the most valuable because it shows subtle adularescence – which is nothing but an internal glow, which is shown in gemstones such as the moonstone. In the context of birthstones, chalcedony has been found to be the May birthstone, even historically. And it is still a zodiac stone for Gemini. Certain chalcedony varieties are also gifted as presents on wedding anniversaries. For instance, onyx is considered the traditional 7th-anniversary gemstone, carnelian is considered the 17th-anniversary gemstone, and lastly, chrysoprase is considered the 18th-anniversary gemstone.