Opals and quartz are both comprised of SiO2.  The difference is that opal is hydrated, or contains water as a component of its molecular structure.  Quartz has a definite crystalline structure while opal is amorphous.  Opal and quartz are found in many parts of the world.  Monster-sized quartz crystal are found in parts of Arkansas.  Hugh Miser, one of the world’s most famous geologists, had perhaps the best collection of Arkansas quartz.  Some of the crystals are taller than a man and weigh hundreds of pounds.  Miser gave half of his collection to the University of Oklahoma and half to the University of Arkansas.  If you’re ever in Norman, Oklahoma or Fayetteville, Arkansas, I strongly advise stopping for a look.  You won’t be disappointed.

http://www.ericwilder.com