Hey! Sorry about your missing rock! If the rock isn't in the chest, be sure to check all the blue items spots to see if you placed it there.
Also, sometimes an item is pressed up against the side of the chest closest to the player and hard to see, so just make sure u get a good look in there.
If you really can't find the rock, then you can either clear your saved data, or I can just give you the clue to the red safe, which that puzzle chain eventually leads to.