Feldspar is a group of minerals that are very important in rock formation, accounting for over half the earth's crust!  There are a number of varieties that are used in jewelry.  The most important are andesine, sunstone, amazonite, moonstone, and labradorite, the latter known for their phenomenal iridescence and adularescence. 
Amazonite was used by the ancient Egyptians for the carving of deities.  The stone was considered a catalyst between the living and the gods.  Moonstone was thought to drive away sleeplessness. 

Feldspars are divided into two types. 

K-feldspars (potassium feldspars). 

Plagioclase feldspars (an isomorphous series between albite and anorthite).