They may or may not contain adjacent minerals in the sedimentary rock, such as clay or feldspar. In Arkansas, the best known quartz is that which formed from hydrothermal fluids in veins that fill fractures in differing types of host rocks. Minable veins are present in either sandstone or shale. Shale-hosted veins are predominantly milky quartz, but tend to occur as larger individual veins than those in sandstone. Milky quartz is the most common variety, making up the great bulk of all veins.
Rock crystal is much less common, although in places it is very abundant.
Only the core of such veins, along with isolated pockets scattered throughout the vein, produce any rock crystal. The major commercial deposits of rock crystal are usually sandstone hosted. They tend not to be one single vein, but a complex series of veins that follow the fracture patterns of the rocks that were broken and shattered by the mountain-building processes. Deposition of quartz took place several times, often interrupted by breakage and refracturing of the host rock. The major deposition of quartz in the Ouachita Mountains of Arkansas and Oklahoma took place during the Late Pennsylvanian to Early Permian Periods, some 300 to 250 million years ago.