Aventurine is a form of quartz. It has flecks of other minerals embedded giving aventurine a glistening effect also called "aventurescence". Flecks of fushsite - a chromium rich mica - will make the classic green aventurine. Flecks of muscovite and ilmenite will produce a blue aventurine. Because of the addition of these flecks, the hardness of aventurine is only 6.5 - a little lower than a quartz crystal.