Quartzite has a conchoidal fracture because it's essentially solid quartz cementing remnant quartz sand grains. Sandstone will generally exhibit preferred parting along cross-bedding or top-set laminations.
Your question is a false dichotomy... quartzite (metamorphic rock) is composed of quartz (mineral). Quartzite is formed when a quartz arenite (sandstone) is metamorphosed under high temperature and pressures. The original quartz grains recrystallize, creating an interlocking mosaic . This explains why, at the microscopic level, you are having difficulty distinguishing the two. They are identical!
