What best defines a rock?

• A. A mineral
• B. A natural material composed of one or more kinds of minerals
• C. A natural material made entirely of crystals
• D. A material formed from organic matter

Answer: (B)

Rocks are natural solids that form from minerals. A mineral is a naturally occurring inorganic substance with a definite chemical composition and a crystalline structure, while a rock is an aggregate made up of one or more minerals. That combination—natural origin and mineral composition—is what defines a rock.

Some choices miss the mark because a mineral by itself isn’t a rock, since a rock is a collection of minerals. Saying a rock is made entirely of crystals is too strict, because not all rocks are perfectly crystalline (some have glassy or poorly crystalline components). And while some rocks can originate from organic material, that isn’t the defining feature of rocks in general.