Quartzite is sandstone squeezed so hard it breaks straight through its grains
Take pure quartz sandstone, bury it inside a colliding mountain belt, and heat and pressure fuse its grains into an interlocking mosaic. The result, quartzite, is so tightly knit that a hammer blow cracks through the crystals rather than around them. That toughness explains its bare ridges, Stone Age tools and trendy kitchen counters.
Geologists define it as mostly quartz; the British Geological Survey requires at least 80% by volume. Pure specimens are white to grey, while traces of hematite tint many pink or red, and other minerals can add yellow, green or blue. As the grains recrystallise, the original sandy texture and layering vanish. By 1941 geologists had also noticed sedimentary rocks cemented so thoroughly with extra quartz that they look identical without a microscope, now called orthoquartzite to separate them from the metamorphic kind. The dividing line is the moment strained grains start being replaced by small new ones.
Because weathering barely touches it, quartzite tends to stand up as ridges and hilltops. Almost pure silica offers little to build soil, so those ridges are often bare rock with scant vegetation. Examples include Errigal in Ireland's County Donegal, the Stiperstones ridge in Shropshire, the white La Cloche Mountains of Ontario, and Monte Binga, Mozambique's highest peak. The town of Quartzsite in Arizona takes its name from the rock in nearby mountains. Ancient North American quartzites are puzzlingly pure for sediments eroded from igneous rock, possibly reflecting odd weathering conditions on the early Earth.
People have used it since the Paleolithic, alongside flint, for stone tools, and prehistoric inhabitants of the southeastern United States shaped mortars from it. Today it is quarried on a large scale; one site near Austertana in Norway produces 850,000 tonnes a year. It serves as crushed stone for roads, as a silica source for silicon and silicon carbide, and increasingly as countertops, since it resists stains better than granite.
That same composition carries a workplace hazard. Cutting, grinding or polishing it can release fine crystalline silica dust, a recognised human carcinogen linked to lung diseases including silicosis.
Source: Quartzite
