In India, it was said to bring good fortune, and that holding a moonstone in one’s mouth during a full moon would enable a person to foresee his or her future.

Moonstone is a type of feldspar. It is fashioned into cabochons, and depending on the angle at which it is viewed, fine- quality material will display a bluish band of milky light that resembles moonlight. There are many types of feldspar, making up about 50 percent of the weight of the Earth’s crust, but very few of these can become beautiful gemstones. Moonstone is seen in Roman jewelry from about 100 A.D., and has since been popular in Art Nouveau jewelry of the late 19th century, as well as in jewelry by Cartier and Tiffany’s in the 20th century. Along with peridot, moonstone is often seen in antique jewelry, and it is an attractive gemstone that is well suited to summer. Moonstone is the birthstone for June, along with pearl and alexandrite, and it has become popular in Japan in recent years. It has a hardness of 6 to 61/2 on the Mohs scale of hardness; being softer than quartz (hardness 7), it scratches easily. It also cleaves easily, so the wearer must take care to avoid impact, especially when it is worn in rings.

When moonstone is fashioned into a cabochon (a convex surface), a mysterious bluish milky light or sheen (adularescence) appears within the colorless and almost transparent stone. This adularescence in moonstone is caused by the diffusion of light that is reflected from planes within the stone, with a most attractive blue effect appearing when the planes are thin. When the planes are thick, the effect is a less-attractive white. When moonstone is cabochon cut with a high dome, the band of light is concentrated into a thinner, shaper line that resembles a cat’s eye. However, in extreme cases the band of light becomes too thin and less distinct. A moonstone like the one in the photograph, with a strong blue color and a sheen that extends from one end to the other, is highly valued.