Here's why:

* Fine-grained: Shale is formed from very small sediment particles, giving it a smooth, fine-grained texture.

* Hard: While not as hard as granite, shale is still relatively hard and can be difficult to break.

* Splits into thin layers: The most defining characteristic of shale is its ability to split into thin, flat layers called laminations. This is due to the way the sediment particles were deposited and compressed over time.

Other rocks that could potentially fit your description, though less likely, are:

* Slate: A metamorphic rock formed from shale. It is even harder and splits into thinner layers than shale.

* Mudstone: A less compact version of shale that is slightly softer and may not split as easily.

If you can provide more information about the rock's color, texture, or location, I might be able to give a more definitive answer.