Which rock is harder, marble, limestone or granite, and why?
It goes granite, marble, limestone, and it comes down to simple minerals. Granites have large amounts of quartz (SiO2) and other hard silicate materials (quartz = 7 Mohs). Limestone on the other hand is basically just a huge pile of non-interlocked but cemented calcite (CaCO3), which is the definition of a Mohs 3. Marble is also calcite, but as it went through metamorphism (from limestone), its crystals became interlocked, which gives it a better hardness value than limestone (pumped up to about 4.5) — but obviously still lower than granite. This is why marble is the favored sculpting stone. It’s harder than limestone, so it wont’ just fall apart beneath a chisel, but it’s not going to require a freaking jackhammer to carve the statue of David.