actually I asked a jeweler friend and the most common is light blue but apparently it comes in several colors which I didn't know, ya learn something new everyday :-)
here, see http://www.jtv.com/on/demandware.store/Sites-jtv-Site/default/Search-Show?q=apatite&srule=default&sz=96&showProducts=1 they have very litte of the darker blue ones tho