According to fellow artist and friend Giorgio Vasari, the statue was not signed, but Michelangelo changed his mind after overhearing his work had been credited to a rival. He snuck back in to chisel his name on the sash across Mary’s chest.
Ignominy kept Michelangelo from signing future works of art.
All this creamy white marble comes from one place, Carrara, Italy. Several years ago, a student artist at the Academy of Art University told me one cubic foot costs $1,000.
The marble mines, at elevation 3,000′, give employment to about 6,000 men and boys and are the sole support of a population of nearly 75,000 in this part of Italy. https://www.scientificamerican.com/article/the-marble-quarries-of-carrara/