Skillman, Smylee;
I use several tools to engrave horn, depending on the look of the line I want. For very bold lines, I use a square push graver that is sharpened to about a 45 degree face angle. For less bold lines, I use a push knife graver, again kept very sharp and with a 45 degree face angle. For a lot of other lines, I use a sail maker's needle (seen in some of these pictures) a scribe, and a surgical scalpel. What I use not only depends on the size of the line I want but the grain coarseness and direction on any particular horn surface. (For me, this is much easier to do on ivory). Sometimes I can't get a tool to go where I want it to go on horn no matter what I try. I will post another small priming horn I just did, with a more complicated design, that has more variety in the types of lines I cut.
Hope that answers the questions.
Dave C