I use perlite when I am out of granite. Sift and wash just like the other ingredients.

Growstones, Volcanic Rocks, Slate, Perlite, Grit all can replace the granite component. If not, it's always good to have more (you can have more than 2 components) of these ph neutral friendly typed rocky ingredients for your mix when you grow cacti and succulents.

River stones can sometimes ( often ) be non porous if it has a smooth surface. As a top dressing it would be okay. Smooth surfaces I wouldn't use it as a part of the soil. Construction sand ( next to but not tthe play ground sand at BBS Home Depot ,Lowes ect. ) that's been sifted out from it's finer powder would be porous enough for use as a part.

As Nomen stated, smooth river stone isn't very useful. It doesn't hold water or nutrients and could be considered "wasted space" in your pots. There are better options.

Each of the inorganic components that Bernard has mentioned has slightly different properties. Here is an article that may be helpful for you:

Art of Bonsai Soils Part 2: Mineral Components

Splendid,,thank you all for the info!