A small piece of rock that has broken off a larger rock is called a fragment or a rock fragment.

Here are some other terms that might be used depending on the context:

* Debris: This term is often used to describe broken rock fragments that are scattered around.

* Shatter cone: This refers to a cone-shaped fracture in rock created by a high-impact event, like a meteorite impact.

* Clasts: This term is commonly used in geology to refer to fragments of rock that are embedded in a sedimentary rock.

The specific term used will depend on the size, shape, origin, and context of the rock fragment.