How about Lava Rock? Why must it be smooth?
Also any dark metamorphic rock, such as a basalt.

I visited the Japanese garden in Seattle and learned the Japanese way to position rocks is to bury them 2/3 deep. It must be why the Japanese gardens never look like a rubble yard.

Blasted quarry rock can be used to make rock outcroppings.

Rounded rock works best with water.

I wouldn't mix the two haphazardly.

Mike

I have a small feature garden out the front our our place. I used dark brown volcanic rock with a stream of white rocks through the middle for contrast...