Quartzite forms in two different ways. In the first way, under the high pressures and temperatures of deep burial, sandstone or chert recrystallizes resulting in a metamorphic rock. A quartzite in which all traces of the original grains and sedimentary structures are erased may also be called metaquartzite. This Las Vegas boulder is a metaquartzite. A quartzite that preserves some sedimentary features is best described as a metasandstone or metachert.
The second way involves sandstone at low pressures and temperatures, where circulating fluids fill the spaces between sand grains with silica cement. This kind of quartzite, also called orthoquartzite, is considered a sedimentary rock, not a metamorphic rock, because the original mineral grains are still there and bedding planes and other sedimentary structures are still evident.
The traditional way to distinguish quartzite from sandstone is that quartzite fractures across or through the grains whereas sandstone splits between them.
For more photos see the Metamorphic Rocks Gallery.
Other galleries:
Fossils
Geologic Features and Processes
Glaciers and Ice
Landforms
Minerals
Rocks
Geology and Society