Marble is a metamorphic rock resulting from the heating of limestone or dolostone. The heating may be regional due to the subduction of a limestone or may be the result of contact metamorphism where a magma intrudes into a limestone, resulting in partial melting.
Marbles that came from very pure limestone are white.
Note that marble has several properties in common with the calcite or (more rarely) dolomite which are its primary components. Marbles are highly susceptible to acid etching, will decompose under contact with even a weak acid, and will vigorously effervesce in the presence of a strong one such as muriatic acid. Marble is also relatively soft, having a Mohs hardness of 3, indicating that it is easily scratched by most materials including common metal utensils.