Here's why:

* Foliated rocks have a layered or banded appearance due to the alignment of mineral grains. This alignment is caused by pressure and heat during metamorphism. Examples include slate, schist, and gneiss.

* Nonfoliated rocks lack this layered structure. Their minerals are randomly arranged, often forming a granular texture. Examples include marble, quartzite, and hornfels.

So, the texture of a rock, which is determined by the arrangement of its minerals, is the key factor in whether it's classified as foliated or nonfoliated.