Marble is simply limestone that has been compressed and/or heated deep within the earth's crust. Unlike granite, marble was never molten rock, but it may have been heated and squeezed enough for the limestone grains to bend and flow.
Marble is also made up of only one mineral -- calcite -- a relatively soft mineral with a hardness of "3". Calcite is a common mineral -- all limestone and nearly all seashells are made of it, but it does not occur in granite. Because marble is made entirely of calcite, it is much softer than granite.
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