Here's why:

* Sedimentary Rock: Rocks like limestone and dolostone are formed from the accumulation and cementation of sediments (like shells, coral, and other marine organisms).

* Metamorphism: When sedimentary rocks are subjected to intense heat and pressure deep within the Earth's crust, they undergo a process called metamorphism. This transforms their mineral composition and texture.

* Marble: Limestone and dolostone, when metamorphosed, become marble. The heat and pressure recrystallize the calcium carbonate (in limestone) or calcium magnesium carbonate (in dolostone) into a new, denser form.

So, while the original rock might have been sedimentary, the marble itself is considered a metamorphic rock.