Check the relevant Wikipedia page.

When the answer to a clue is a person's name, use their last name.

When we were writing puzzles for the Earth round, answers were already determined to be particular rocks, including limestone. Meanwhile I had already come up with the idea for this puzzle. There are only 365 answers this puzzle could have taken -- one for each day of the year -- and fewer if you consider which dates can be expressed in 12 letters (such as "JANUARYSEVEN"). And coincidentally, one of those was limestone!