The ancient Romans theorized that moonstone, with its unearthly shimmer, was formed from frozen moonlight. This appealing gem variety does shine with a cool lunar light but it is the mineral feldspar, quite terrestrial in origin. The shimmer, which is called schiller or adularescence, is caused by the intergrowth of two different types of feldspar, with different refractive indexes.
Moonstones come in a variety of colors. The body color can range from colorless to gray, brown, yellow, green, or pink. The clarity ranges from transparent to translucent. The best moonstone has a blue sheen, perfect clarity, and a colorless body color. Another related feldspar variety is known as rainbow moonstone. In this variety, the sheen is a variety of rainbow hues, from pink to yellow, to peach, purple, and blue.
Fine moonstone is quite rare and becoming rarer. It is mined in Sri Lanka and Southern India. The rainbow variety can be found in India and Madagascar.
Moonstones are usually cut in a smooth-domed oval cabochon shape to maximize the effect. Sometimes they are carved to show a man-in-the-moon face.