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Which rock is harder, marble, limestone or granite, and why?

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Which rock is harder, marble, limestone or granite, and why?

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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.

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