It depends on what you mean by 'harder to sculpt'. You might want to think of the processes involved in sculpting and what happens to the scuplted piece. The size of the piece might come into your discussion and what the sculptor might be trying to achieve.
Marble is a metamorphic rock usually formed by regional metamorphism. It comes in various colours and also variations in hardness.
Limestone is a sedimentary rock which can come in large homogeneous blocks, e.g. Portland Limestone.
Limestone and Marble are both used in scuplturing. Limestone becomes Marble under heat and pressure. Classify each rock, and tell which would be harder to scuplt.
4 answers
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