Silica is dissolved and then reprecipitated, crystallizing as quartz during this process. Usually the crystals are free-floating in the matrix rock and never reach very large size. Some quartz crystals present in the matrix of dolostone or limestone formed in this manner. 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.