Mineralogy
Studying minerals can be useful for figuring out certain things about a rock. Sometimes the shape or size of the mineral can tell something about the rock as well. For example, minerals in igneous rocks can help figure out how long the rock took to cool down (turn from lava into a rock). Larger minerals mean the rock cooled slowly (probably underground). Smaller minerals mean the rock cooled down quickly (probably above ground, like from a volcano). The type of mineral in a rock can also tell what kind of a rock it is, or what has happened to the rock since it formed. Many rocks are named based on what kinds of minerals they have.