granite

Granite is an igneous rock, which means it formed from magma, or melted rock. It forms deep inside the Earth under a mountain or volcano when melted rock cools or crystallizes into solid rock. Over time, wind, ice, and water wear away at the mountain or volcano above it, and the granite is exposed to the surface. Granite is used as a building material because it's a hard stone.

Definition: a rock that forms when magma cools deep in the Earth's crust
Type: igneous
Appearance: typically light and dark speckles, with large crystals
Primary minerals: feldspar, quartz, and mica
Cool fact: Built in the second century B.C. and still standing today, the Great Wall of China is made of granite and other rocks.

Image credits: courtesy of AMNH; Christine Tappen: courtesy of AMNH.