One stone. Pyrite is commonly known as "Fools Gold" due to it's gold like propertites.  I personally use pyrite and tiger's eye in combination to protect myself and my car by both wearing the stones as well as keeping them in all of the compartments of each door in my car. Known uses for Pyrite: 
Boost energy levels and promote physical strength and vitality 
Enhance mental clarity and aid in problem-solving 
Work as a powerful protection stone that shields and protects from all types of bad vibrations and/or energy 
Stimulate the mind and improve memory, making it easier to recall important information when needed 
Relieve anxiety and frustration, and as a mirror to the self, reveal the causes behind these emotions and promote a search for solutions 
Help one recognize the universe’s purity