This is the very question that has been plaguing our industry these days. The scientific definition of quartzite is a metamorphic rock made almost entirely of the mineral Quartz.
Its geologic cycle starts from simple grains of sand, compressed over time to create sandstone. The sandstone then gets compressed and heated again causing the sand grains to fuse together forming the durable rock we call quartzite. This is not to be confused with the manufactured product people are now calling quartz.