Here's why:

* Igneous rocks are formed from the cooling and solidification of magma or lava.

* Fine-grained rocks, also called aphanitic, are formed when magma cools and solidifies quickly. This rapid cooling doesn't give the mineral crystals time to grow large enough to be seen with the naked eye.

Examples of fine-grained igneous rocks include:

* Basalt: A dark-colored volcanic rock

* Rhyolite: A light-colored volcanic rock

* Andesite: An intermediate-colored volcanic rock

In contrast, coarse-grained or phaneritic igneous rocks have larger crystals that are visible without magnification. These form when magma cools slowly, giving the crystals ample time to grow.