Most of the time, this material was rapidly eroded away. But some times, patchy deposits of this were buried under sand and preserved. The entire package of sand, gravel and clay was buried and metamorphosed. The sand and gravel formed quartzite and the clay and silt lenses formed the pipestone. Pipestone consists of a fine-grained intergrowth of sericite, kaolinite, diaspor, hematite and pyrophyllite.
Quartz grains are also present, but the more quartz, the harder the pipestone is to carve.
Sericite is a fine-grained mica. Kaolinite is an alumnium-rich clay mineral. Diaspor is an aluminum hydroxide. Pyrophyllite is an aluminum silicate. These minerals tend to be very soft.