Azurite

Azurite
Called “The Stone of Heaven”
Resonates to the exact frequency of the third eye 
Stimulates the mind centers
Encourages study for new and challenging subjects
Enhances focus, concentration, memorization and retention of information
Clears tension and confusion
Stimulates the intellect,
Awakens the development of psychic and intuitive abilities
Brings inner vision into alignment with spiritual guidance
From Peru
 
These are priced by weight and not necessarily by size.   This is a stone I personally work with on a daily basis.  It's one of my favorites for focus.