Melissa This is a sample of quartz crystals. Quartz is a mineral and is different from quartzite, which is a metamorphic rock formed by altering quartz sandstone by heat and pressure. These crystals are purple because when they formed, there was small amounts of another mineral, iron, that caused the coloration. This variety is called 'amethyst' and is considered a gem stone. Although amethyst is found in the Black Hills, it is rare enough that I would not suspect that it would be for sale.
Most Amethyst sold locally is brought in from another location or country.
Amethyst is only found in one mine in the Black Hills and also occurs in the center of some agates in the southern Hills. Rose quartz, which is pink or red, is abundant in the Black Hills. The red color comes from small amounts of a rare metal, titanium, incorporated in the crystal structure.