Moonstone is technically a non-crystalline gemstone, composed of a blending of varying amounts of two different minerals, Orthoclase and Albite.  Both of these minerals are variations on basic Feldspar, composed of Aluminum, Silicon and Oxygen, with a basic formula of AlSi₃O₈.  Orthoclase adds Potassium to the mix, with a formula of KAlSi₃O₈, while Albite adds Sodium, with a formula of NaAlSi₃O₈. 

These two minerals, in extremely thin, translucent plates, produce the bluish or silvery sheen which gives the stone the look of moonlight.  This sheen is called “adularescence” or “schiller.”  Thicker plates produce a silvery sheen, while thinner ones create the bluish iridescence in the most beautiful of the stones.  Still other Moonstones, in a variety of colors such as blue, brown, green, grey, white or even yellow.  These generally are comprised primarily of Albite, with little or no Orthoclase, and are less translucent than those with greater proportions of Orthoclase.

Labradorite, sometimes called the Black Moonstone, is a variety of Feldspar including both Sodium and Calcium added to the basic Aluminum-Silicon-Oxygen mix.

Moonstone has always been revered because of its lunar attraction. It was believed that the cycles of the moon were reflected in the iridescent shimmer (schiller or adularescence) in the stone. It has always been considered a “feminine,” or “Goddess” stone.
The Magickal and healing properties of the Moonstone have long been appreciated, especially in regards to healing for women.  Because of its Lunar connection, the Moonstone has been seen as relating to the three faces of the Goddess (Maiden, Mother and Crone.)  It has been used as an aid during childbirth, with a mother holding tightly to a Moonstone during labor.  It also is helpful with a variety of other “female problems,” such as menstrual cramps, and helps to balance a woman’s hormones during menopause.

Men, too, however, receive great benefit from this stone. 

The Moonstone helps to heal problems with the pancreas, pituitary gland stomach, spleen and the lymphatic system.

Emotional benefits derived from Moonstones include emotional balance and stability, stress and anxiety reduction.  It helps to clear mental confusion, and is a potent aid in making calm, logical decisions.

Moonstone is a powerful tool for enhancing psychic abilities, and is useful for increasing psychic sensitivity.  In meditation, the Moonstone helps one go into deep trance states.  It is especially potent when used in meditations regarding the Goddess in Her Diana (Maiden), Selene (Mother) and Hecate (Crone) guises. 

This is always a stone of good fortune, especially when given as an exchange between lovers.  It helps to insure enduring love, and helps reunite loved ones who have parted in anger.  It also protects those who travel on water.
Some dislike wearing the Moonstone, because they fear its tendency to reveal the mood and personality of the bearer by changing transparency and color
Moonstone aids in all forms of divination (Tarot, Runes, I-Ching, etc.)