Moonstone is a chatoyant variety of feldspar that occurs in all the colors of the full moon—silver white, golden peach, and slate blue. There are also colorless varieties, known as adularia or "rainbow moonstone", which are especially prized. Regardless of color, all moonstone exhibits a silvery blue gleam that appears to come from just below the surface. The best grades of moonstone come from India, where according the some accounts, the stone was held to be sacred.
Whether moonstone was known in the Middle Ages is uncertain. Pliny's description of selenite would indicate at least a second hand knowledge of the stone. But his description could also apply to gypsum, a fairly common mineral. On the other hand, Albertus Magnus' description of the same stone a thousand years later is vague and confused. Possibly, the name "moonstone" was applied to a variety of gemstones, such as cat's-eye, opal, and star sapphire.
Chemically moonstone is a member of the feldspar group (potassium aluminum silicate: KAlSi3O8). There are related varieties with similar chemical composition. With a hardness of '6' it is somewhat soft for a gemstone. Also, like all feldspars, moonstone exhibits cleavage in two directions but is durable enough when cut en cabochon to be used in most jewelry applications. Moonstone beads are especially popular.
A closely related stone is blue moonstone. Actually a variety of laboradorite (plagioclase Ca/NaAlSi3O8), it is nearly transparent and has an intense blue schiller. The term "rainbow moonstone" can apply to this stone as well.