Quartzite

Quartzite is a hard, non-foliated metamorphic rock formed almost entirely of quartz. It begins as quartz-rich sandstone, which under heat and pressure recrystallizes so that the individual sand grains fuse together into a solid, interlocking mass.

The defining feature is that quartzite breaks through the grains rather than around them, producing a smooth, glassy fracture — in contrast to sandstone, which crumbles along grain boundaries. Pure quartzite is white or grey; iron oxides add pink, red, and yellow tints.

Its durability makes it a ridge- and cliff-forming rock in many mountain ranges, and a popular natural stone for kitchen counters.

Quartzite forms by regional or contact metamorphism of quartz sandstone. As temperature and pressure rise, silica recrystallizes and the pore spaces between grains close, welding the grains into a dense, interlocking quartz fabric.

Because quartz is chemically stable, no new minerals typically form — the rock simply becomes harder and denser. Any clay or iron impurities in the original sandstone may form thin micaceous layers or color streaks.

Quartzites are found in ancient cratons and fold belts worldwide, often as resistant ridges where softer surrounding rocks have eroded away.

The key test is hardness and fracture: quartzite is Mohs 7, scratches glass and steel easily, and breaks across the grains giving a glassy, conchoidal-to-splintery surface. Sandstone, by contrast, is softer and crumbles into sand.

It does not fizz in acid (distinguishing it from marble) and has a sugary, sometimes glassy sparkle on fresh breaks. Streak is white.

Look-alikes: Marble is much softer and reacts with acid. Massive vein quartz looks similar but lacks the relict granular texture. White marble can mimic white quartzite visually, but the knife and acid tests separate them instantly.

Quartzite's extreme hardness and resistance to weathering make it valuable as crushed stone for railway ballast and road aggregate, and as a durable dimension stone for paving, walls, and increasingly for kitchen countertops, where it outperforms marble for scratch and etch resistance.

Very pure quartzite is a source of silica for glassmaking, ferrosilicon, and silica brick used in furnace linings.

Decoratively, polished quartzite slabs in white, grey, and pink are popular high-end surfaces. It carries no significant gem value but is occasionally tumbled.