Quartzite is a common, crystalline-textured, intermediate- to high-grade metamorphic rock. It forms by metamorphism of quartzose sandstones or siltstones. Quartzite can be entirely composed of interlocking quartz crystals, or the original sand grains may still be visible. This rock is hard (H = 7), will not bubble in acid (unlike marble), and can be almost any color (see photos of 4 color variants below).
The term "quartzite" has been used in geology to refer to crystalline, quartzose metamorphic rocks and to hard, well-cemented quartzose sandstones that have not been subjected to metamorphism. It is difficult to not call hard, well-cemented sandstones “quartzite” (for example, the Clinch Quartzite in the Appalachian Mountains & the Eureka Quartzite of the Great Basin in western USA). But the Clinch and Eureka aren't metamorphic rocks. The term metaquartzite has been used by some geologists to refer to crystalline-textured, quartzose rocks that have been metamorphosed (see samples below). This implies that “quartzite” be restricted to well-cemented, non-metamorphosed sandstones. I don't often see the term metaquartzite used in the geologic literature.
Quartzite (metaquartzite), 3.1 cm across.
Quartzite (metaquartzite), 3.9 cm across.
Quartzite (metaquartzite), 2.6 cm across.
Quarzite (metaquartzite), 3.0 cm across.