Here's why:

* Fine-grained: Phyllite is a metamorphic rock formed from shale, which is a fine-grained sedimentary rock. This means the mineral grains in phyllite are very small.

* Wavy or Crinkled: The defining characteristic of phyllite is its wavy or crinkled surface, which is created by the alignment of very fine mica minerals during metamorphism. This gives the rock a sheen or shimmer.

Here are some other characteristics of phyllite:

* Foliated: Phyllite is a foliated metamorphic rock, meaning it has a layered or banded appearance due to the alignment of its mineral grains.

* Often lustrous: The aligned mica minerals in phyllite can give it a silky or pearly luster.

* Can be soft: Phyllite is relatively soft and can be scratched with a fingernail.

While other characteristics like color and hardness might be relevant, the wavy or crinkled texture is the most unique and defining feature of phyllite.