What is a quartzite in geology ?

Quartzite : definition

Quartzite is a term that can designate two different rocks, having in common a very high silica content.

It can relate first to a sedimentary rock made up of grains of detrital quartz, cemented by newly formed quartz, which gives it a very great hardness. It can also be a metamorphic rock resulting from the recrystallization of sandstone or siliceous rocks initially made up of amorphous silica and chalcedony (radiolarite for example).
To avoid confusion, it is preferable to use the term metaquartzite to refer to quartzite of metamorphic origin.

It is a rock that is used in ornamentation and jewelry, it is sold under the name of aventurine (see photo).