I saw a floor once where the
tiler marked the all the cuts before he cut and stuck them with a number in non permanent marker on the reverse side .
Came back next day and the numbers had all leached up through 30mm thick stone and were clearly visible on the top.
Very difficult to get out but oxybrite crystals worked in the end from supermarket.
make sure you do a test first to see if effects travertine though.
incidentally pencil can be very difficult to get off some stones, but there is cleaner called brickie which is non acidic which you can get from builder centre that works very well. again test first
johnny