I was only disagreeing on the possible terminology. I wasnt referring to the (probably) machine made ones, which looked like they were virtually laser cut (ie too perfect).

Im not convinced that a knife would regularly engrave that deeply. If the knife was heavy and sharp enough it would leave a clean line. Im still suspicious that some of these have been done with an electric engraving tool.

Another possible factor that bugs me. I do a bit of wood carving from time to time. I bought some relatively fresh cut oak about 5 years ago which was plain and simple kiln dried for a couple of hours (standard).
It was really easy to cut and carve leaving a perfect smooth finish.
A couple of weeks ago I finally got back to start carving something else. I took a piece of the same wood that had been sitting in the garage for 5 years. It is now hard as nails. Same tools, same attempted carving. Impossible to get a smooth finish without sanding. Compares to your smooth/rough cuts on the chips.
I wonder how much harder the ivory becomes over time, and therefore how much harder it is to 'cut' a smooth line in it?