Here's why:

* Metamorphic: Marble is formed when limestone, a sedimentary rock, is subjected to heat and pressure deep within the Earth's crust. This process transforms the limestone's structure and composition.

* Rare: While marble is not incredibly rare, high-quality marble with unique patterns and colors is highly sought after and can be considered rare.

* Beautiful: Marble is known for its beautiful, often veined and swirling patterns, as well as its smooth, polished surface.

Let me know if you'd like to know more about marble or other metamorphic rocks!