"Hard rock" doesn't refer to a specific type of rock in the geological sense. It's a musical genre, characterized by its heavy sound, distorted guitars, strong vocals, and often fast tempos.

However, some rock formations are known for their hardness, such as:

* Granite: A hard, igneous rock commonly used in construction.

* Basalt: A dark, volcanic rock known for its durability.

* Quartzite: A metamorphic rock formed from sandstone, known for its extreme hardness.

If you're interested in learning more about specific rock types, you can search for "types of rocks" or "rock classification" online.