It's a metamorphic rock made of calcite or dolomite crystals that were subjected to intense heat pressure in the Earth's crust. In the past, marble was mined by putting sticks into mountain rocks that had been fractured.
You can see the rocks within by splitting it apart. Even though it's more difficult to mine, marble boasts more attractive patterns because of its rarity and fragility. As a result, it is more expensive than granite. Although marble is expensive, many people still use it as a countertop. That is, assuming you have the cash to spare.