Explainer: What is a rock? The definition is caught between a rock and a hard place. Credit: Jodie Griggs / Getty Images. Everyone is familiar with rocks. It’s hard not to be. They’re everywhere – we walk on them, skip them across ponds, dig them out of our gardens, and stare appreciatively at them as they form mountains and canyons. So, we all know what a rock actually is, right? Turns out rocks are pretty hard to define The definition of a rock is a bit woolly, often contradictory, and hard to pin down – even for geologists. In the broadest sense, a rock is a collection of other things. Officially, it is a collection of one or more types of minerals, held together in a solid mass.