Igneous rocks that crystallize inside the Earth are called intrusive or plutonic rocks.

Here's why:

* Intrusive: This term emphasizes the way these rocks form—they intrude into existing rock layers.

* Plutonic: This term is named after Pluto, the Roman god of the underworld, and refers to the fact that these rocks form deep within the Earth's crust.

Here are some examples of common intrusive igneous rocks:

* Granite

* Gabbro

* Diorite

* Peridotite

Let me know if you'd like more information about these rocks!