Final answer:
Marble forms when limestone is exposed to heat and pressure, resulting in a harder rock with a white color.
Step-by-step explanation:
When limestone is exposed to heat and pressure, it undergoes metamorphism and forms marble. Marble is a metamorphic rock that is usually white in color and is composed of recrystallized carbonate elements. This process occurs when the limestone is subjected to high temperatures, compression, or pressure, resulting in a harder rock.