Fluorite 
Fluorite is a stone used for mental clarity. It helps improve concentration and brain power, this crystal will help you focus and get done what you need to do. Perfect crystal for students as it acts as a learning aid: stimulating the mind and helping clear distractions. An affirmation to go with this stone is ‘My mind is clear and I am focused.’ 
This crystal is accosiated with the crown, third eye, throat and heart chakras.