I started scrimshawing on ivory and found that a coarse needle or fine awl worked very well on that material, however, such tools are more difficult to use on cowhorn and that is why many recommend using exacto knives and the like. My arthritus has limited my skills these days, but I still do an occasional horn...also on well-polished ivory one can ink the surface first and "draw" on the inked part, then re-ink and scrub off the background--this does not work well on surfaces with small scratches and pits. On most horns, for example, I draw on the bare surface, then ink. I typically use black india ink.