Quartz crystal, or silicon dioxide (SiO2), is one of the most prolific minerals in the world, according to the Ouachita National Forest website. The Ouachita National Forest in Arkansas contains a large quantity of high-quality quartz crystals. Visitors to the Ouachita National Forest can locate this naturally occurring quartz crystal with the help of the rangers. Miners, known as "rockhounders," must get permission to mine the forest.
Pyrite is a crystal that is officially known as "iron sulfide" and commonly known as "fool's gold." The National Park Service cites the Cabin Branch Pyrite mine in Prince William Forest Park near Washington, D.C., as a former mining site that has been reclaimed. Today pyrite is used in ordinary items such as soap and light bulbs.
Find purple amethyst crystals in national forests such as Beaverhead National Forest in Montana. There is a large supply of both amethyst crystals and quartz crystals in the forest's Crystal Park. Thousands of people visit the park to dig for crystals every year.