Yes, an old petrified dinosaur egg. lol. Hard to say how old, if it was in a hot spot possibly a week, probably more to cause that type of hardening, probably infertile.
Once in a while you can find one random stray egg that was tucked out of sight while the girls are looking for their spot and one slips out before they get to the lay box.
I meant that it could have been from last month [ was her entire tank cleaned + every single thing moved so that you would have found it ? ] left from that laying. I don't know that it would get to look like that after only a few days.