Here's why:

* It starts as a sedimentary rock: Onyx is formed from the deposition of calcium carbonate (CaCO3) in the form of a layered structure. This layering is crucial for the characteristic banding of onyx.

* Metamorphism alters the structure: The layers of calcium carbonate are subjected to heat and pressure, which recrystallize the material, creating the dense, hard, and banded structure that we recognize as onyx.

So, while onyx originates from sedimentary material, the process of metamorphism changes its structure and properties significantly.