Sorry, my mother tongue is not English so I am expected to make some errors here and there. What I used was a grease pencil indeed, but instead of the peel type, this one comes in a set of 12 colors and you screw the top, and the tip comes out more and more.

My daughter, like most teenagers, keeps buying all these stuff for school and at the end of the year, we have a heap of stuff she leaves behind. I dump dive , LOL, and grab really nice stuff out of it. I asked her where she bought them and she doesn't remember ! I would have bought another set 'for her' except I would steal the red, green, and black pencils

Etching tool is OK but may not be practical. It will not make a VERY visible mark for my needs. At least not suitable for my poor eyesight. But it is a great suggestion. For an improviser like me, it may come handy for things other than making marks.