Here's why:

* Basalt is a dark-colored, fine-grained, extrusive igneous rock. Its dark color comes from its high content of iron and magnesium.

* Other volcanic materials can also be dark, but often have a different appearance:

* Scoria is a type of volcanic rock that is often red or black, with a porous, bubbly texture.

* Ash can be black, but it's very fine and powdery.

* Obsidian is a volcanic glass that can be black, but it has a glassy luster.

So, while other materials can be dark, basalt is the volcanic material most likely to appear black.