Usually something like Staedtler 0.1 pigment liner.

Thin marker (0.1 or 0.2) and a transparent nail polish, and usually an acronym of the site + the number of the find from 1 to the last one. #lithicnerd

Thanks! Will follow your work. :) Good luck! And, like Alice said, a beautiful core 😍

How did you treat it? How long and what temperatures? I did some heat treatment experiments as a student but I'm still interested in this and would love to learn more.