Quartzite is a metamorphic rock that forms from the metamorphism of sandstone. Metamorphism is the process by which rocks undergo changes in mineralogy, texture, or composition due to heat, pressure, and/or chemical reactions.

Sandstone is a sedimentary rock that is composed primarily of sand-sized mineral grains, predominantly quartz, cemented together by a matrix of other minerals or mineraloids. When sandstone is subjected to high pressure and temperature over time, the minerals within it undergo recrystallization and reorganization.

During the metamorphic process, the sand grains in sandstone recrystallize and fuse together. The high pressure and temperature cause the quartz grains to undergo changes in their crystal structure, resulting in the formation of quartzite. The recrystallization process fuses the quartz grains together, giving quartzite its characteristic dense and hard nature.

It`s important to note that limestone, obsidian, and shale do not undergo the same transformation to form quartzite. Limestone is a sedimentary rock primarily composed of calcite or dolomite minerals, and its metamorphic counterpart is marble. Obsidian is a volcanic glass, and shale is a fine-grained sedimentary rock. These rocks have different compositions and undergo distinct metamorphic processes that result in the formation of different metamorphic rocks, not quartzite.

In summary, quartzite is metamorphosed from sandstone through the recrystallization and fusion of quartz grains under high pressure and temperature. Sandstone is the precursor rock that transforms into quartzite during the metamorphic process.