Quartzite

Quartzite is a metamorphic rock. Of the three rock families — igneous (cooled from melt), sedimentary (built from cemented grains or fragments) and metamorphic (an existing rock recrystallized by heat and pressure) — quartzite belongs to the metamorphic group. It did not crystallize from magma and it is not a freshly cemented sediment; it is what a particular sediment turned into after being subjected to intense heat and pressure. So to the common question of whether quartzite is igneous, sedimentary or metamorphic, the answer is metamorphic.

The parent rock of quartzite is quartz sandstone. Sandstone is a sedimentary rock made of sand-sized quartz grains held together by a natural cement. When that sandstone is deeply buried or caught up in mountain-building, rising temperature and pressure cause the quartz grains and their silica cement to recrystallize and fuse into a continuous, interlocking framework. The grain boundaries that were obvious in the sandstone become welded shut, which is exactly why quartzite breaks through its grains while sandstone breaks around them. Chemically little changes — both are dominated by quartz — but the texture and strength are transformed.

Quartzite forms by the metamorphism of quartz sandstone, typically where rocks are heated and compressed during the burial of thick sedimentary piles or the collision of tectonic plates. As temperature and pressure rise, the rounded quartz sand grains and the silica that cemented them begin to recrystallize. Silica is redeposited in the spaces between grains and the grains grow together until the original pore space is gone and the boundaries are fused. This welding of grain to grain is what converts a relatively weak, porous sandstone into a hard, non-porous rock in which the quartz forms one continuous interlocking mass.

Because quartz is chemically stable and extremely durable, quartzite resists weathering far better than most surrounding rocks. Over long spans of time the softer rocks around a quartzite body erode away faster, so quartzite frequently stands out as ridges, hilltops and prominent outcrops in the landscape. The color of the finished rock depends on what impurities were present: clean quartzite is white or gray, while traces of iron oxide can tint it pink or red and other minerals can add yellow or tan hues. Sometimes faint relict bedding from the original sandstone survives metamorphism and can still be seen as subtle banding.

The first and best test is hardness. Quartzite is made of quartz, so it is very hard, around 7 on the Mohs scale: it will readily scratch glass and a steel knife blade, and a knife will not scratch it. This high hardness rules out softer pale rocks. Next, check the acid reaction — there should be none. Unlike marble and limestone, quartzite does not fizz when dilute acid is applied, because it contains no carbonate. A pale rock that scratches glass and does not react with acid is a strong candidate for quartzite.

Then examine how the rock breaks, which is the key to separating quartzite from its sandstone parent. Run a thumbnail or fingertip over a broken surface: quartzite feels solid and the grains do not rub off, and a fresh fracture cuts straight through the grains, leaving a smooth, glassy or sugary surface that may glint in the light. Sandstone, by contrast, sheds loose sand grains and fractures around them, leaving a rougher, more granular face. Quartzite is also typically denser and more compact, with little or no visible pore space. If the rock is hard, acid-inert, and breaks through rather than around its grains, it is quartzite.

Quartzite's extreme hardness and resistance to scratching, heat and weathering make it a sought-after stone for surfaces that must endure heavy use. Polished quartzite slabs are popular for kitchen countertops, where the rock's durability and resistance to etching give it an advantage over softer stones like marble. As dimension stone it is cut for flooring, wall cladding, stair treads and paving, and its toughness suits it to high-traffic and exterior applications where a long-lasting, low-maintenance surface is wanted.

In construction and industry, crushed quartzite is widely used as a hard, durable aggregate in concrete, asphalt, road base and railway ballast, where its strength and angularity are valuable. Its very high silica content also makes certain quartzites a raw material for products that require pure silica, and broken quartzite has long served as a rugged building and walling stone. Across all these uses, the same property — a fused, all-quartz framework that is hard and chemically inert — is what gives quartzite its value.