We've added two bottom steps out of cob to our staircase. They have yet to be finished and sealed, unlike the floor itself - which you can see under the steps, with a similar 'paint' to that used on the interior walls, but also the sealant we mentioned before. It's not perfect yet, and will definitely need judicious use of rugs or even slate tiles in heavy traffic areas; but it is beautiful and surprisingly soft!
Here's our staircase, finished on the left - unfinished on the right! The finished wall has a similar paint to outside, lime, tumeric, and a redder clay than outside onto the plaster, plus an eco-friendly 'plaster primer' sealer.