The property of minerals that causes them to split into pieces with uneven surfaces is called fracture.

Here's why:

* Cleavage is the tendency of a mineral to break along specific planes of weakness, resulting in smooth, flat surfaces.

* Fracture, on the other hand, describes the way a mineral breaks when it doesn't have a defined cleavage plane. This can lead to a variety of irregular, uneven surfaces.

There are different types of fracture, including:

* Conchoidal: Smooth, curved surfaces like a shell.

* Hackly: Jagged, sharp, and uneven surfaces.

* Fibrous: Looks like fibers or splinters.

* Irregular: Just a random, uneven break.

Let me know if you'd like more details on any of these fracture types!