Any flat river rock will sharpen steel. I have done this many times with small knives and large knives. I have sharpenned small knives with small flat rocks by putting the edge at a steep angle and just moving the stone in a circular motion on the edge keeping the angle the same. I find I can keep the angle the same by moving the stone than moving the knife. On larger knives, I have used flat rocks and sharpened the blade by pulling away from the edge instead of cutting into it. The reason for this is that even the smoothest rocks have ledges and the edge will hang up and blunt in a second.

I have also used my clothing to sharpen knives. My Jeans, leather belt, canvas backpack will all strop a blade and keep it sharp longer than if you didn't do anything. I once even used sugar cane as a crock stick.