Yeah, I still don’t get it. There seems to be no clues in lab 3 (I don’t normally do reading puzzles, so there’s probably something really obvious that I’m missing).
I couldn't figure it out either, and after reading other posts I saw something I missed - to try to give a better hint without spoiling it:
Read the note in the 2nd box in the 3rd room. What stone is it referring to?
If that's not enough, I'll put the solution at a separate link (idk how to make spoiler text): https://pastebin.com/q9kcg3Sv (but I recommend trying it for yourself)