Selenite is crystalized gypsum.  These stones have a fibrous, "fiber-optic"-like quality. 

Selenite is a stone used with the crown chakra and helps you tune into your guardian angels and spirit guides.  It is said to bring mental clarity and focus to your thoughts.  It can help you to attain spiritual-physical-mental alignment and helps you to detach from drama, allowing you greater understanding of the "big picture".