When limestone undergoes contact metamorphism, it transforms into marble.

Here's a breakdown:

* Limestone: A sedimentary rock primarily composed of calcium carbonate (CaCO3).

* Contact Metamorphism: A type of metamorphism that occurs when hot magma or lava comes into contact with existing rock. The heat and pressure from the magma cause changes in the rock's mineral composition and texture.

* Marble: A metamorphic rock formed from the recrystallization of limestone. It is typically characterized by its coarse-grained texture and the presence of calcite crystals.

So, the heat and pressure from contact metamorphism cause the calcite crystals in limestone to recrystallize, resulting in the formation of marble.