This is quartzite which is is quartz sandstone cooked for a few million years in the earth's mantle, i.e. it is a metamorphic rock. Quartzite is a hard, non-foliated metamorphic rock that was originally pure quartz sandstone (SiO4). Sandstone is converted into quartzite through heating and pressure usually related to tectonic compression within orogenic belts. Quartzite contains 89-98% quartz with iron hydroxides (2-3%), silicon and chalcedony (4-5%). The stone also contains impurities of mica, talc, feldspar, and other minerals.