This is something I don’t use, really. I always end up using whatever I’ve dedicated as my athame to cut things physically, and apparently you’re not supposed to do that.
I do have two knives. One I got from my grandfather on my mom’s side — it’s a simple hunting knife that desperately needs a sharpening. Brown wooden handle, brown leather sheath.
The second is a bit fancier and sharper — it’s from Guatemala and it has a serrated bit (like a bread knife) on the top of the blade, so it can cut twice. I think the sheath is leather but I could be wrong; it’s hard and red, with some designs on it.
The second one is the one I carry around with me when I’m concerned about the safety of the area I’m going to.
I don’t cast circles anymore, and generally when I wish to direct energy I use a wand. However, I have been thinking that I may use a knife for a spell or ritual to banish energies or cut away connections.
Hasn’t happened yet though.