Moonstone

Moonstone was an opaque white form of feldspar that took on a bluish sheen when polished.

Description
These semi-precious stones were used as a material component in the casting of various spells. Legends about lycanthropes said that the mere sight of a moonstone would cause a were-creature to shift to animal form. Though this was not true (in fact, many lycanthropes wore moonstones), magic items that affected lycanthropes or warded against lycanthropy often used moonstones in their creation and ornamentation. Worshipers of Selûne considered moonstones sacred. A typical specimen had a base value of 50 gp.