Depending on the degree of the cracks there are stone type epoxies you can get on the web that can closely match the marble's appearance. They are often used in countertop installations. However depending on the size of the cracks and the size of the slab it may be easier to replace the slab.

the cracks can be filled IF the marble slabs are still firm and solid.

it really depends ... sometimes it makes them look worse depending on the degree of randomness in the marbleing paters and swirls.

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