Clay is sediment and most surface rocks are too.I'm not sure if that would be true.
There certainly are sedimentary rocks. But, there are tons of different types of rocks including granite, metamorphic rocks, igneous rocks, etc. What is most common in any one location would depend on the geologic region, and the forces that created the surface features.
In many cases sedimentary rock crumbles easily. Lava can be porous. Granite can be some of the hardest rock, great to be crushed into small pieces, then tumbled in rivers to make pebbles.