* Definition of Opaque: Opaque materials do not allow light to pass through them.

* Light Interaction with Rocks: The minerals that make up rocks are structured in a way that reflects or absorbs light. Light doesn't have the ability to travel through the dense, solid structure of a rock.

Exceptions:

* Thin slices: If you make a very thin slice of a rock, light might be able to pass through it, particularly if the rock is translucent.

* Crystals: Some crystals within rocks might be transparent or translucent. However, the rock itself, as a whole, will still be considered opaque.

Therefore, while there might be some exceptions, rocks are generally considered opaque materials because light cannot pass through them.