QUARTZITE is a metamorphic rock that originally was quartz sandstone.
Quartzites are produced by intense heat and/or pressure, probably aided
by hot silica-bearing solutions. The quartz grains may be so closely
interlocked that individual grains are no longer recognizable. The rock
fractures conchoidally through both the grains and cement, so the broken
surface, unlike that of sandstone, is smooth and may even be glassy like
quartz.
Color depends upon the amount and kind of impurities present. A
quartzite that is all quartz is white or gray, but iron or other
elements may change the color to shades of purple, yellow, brown, or
red. Quartzite is a very resistant, hard rock and cannot be scratched by
a knife.
Quartzite is abundant as boulders and pebbles in glacial drift of
Illinois, having been brought into the state during the “Ice Age.”