Quartz is one of the most widely distributed mineral on earth. It can be found in many igneous and metamorphic rocks like granite and gneiss. It can also be found in rhyolite, sandstone, shale, quartzite, and schist.There are certain areas on earth where they find the bigest and most valuable ones though. The chart below shows you some of those places:

Amethyst	Brazil, Uraguay, Mexico, Russia, Thunder Bay area of Canada, and some locallities in the USA.
Smoky Quartz	Brazil, Colorado, Scotland, Swiss Alps among many others.
Rose Quartz	Brazil
Rock crystal	Brazil, Arkansas, many locallities in Africa, etc.
Agates	Brazil, Lake Superior region, Montana, Mexico and Germany.

As you can see Brazil is where a lot of the crystals come from. The bigest rock crystal was found in Brazil.

Most good crystals are obtained in the mineral veins where quartz is found.Big quartz crystals are often found near hot springsThey can also be obtained from geodes, granite porphyries, and granite pegmatities.