it says the answer to why the diamond was there won't be recorded
so here it is:
It's actually like this; The diamond wasn't guarded or anything because, let's be honest, who'd expect a diamond with the power to turn you into basically a god would be hidden in a random, abandoned building? First of all, you'd actually have to know about the diamond, which seems to be hard since none of us but the girl knew about it, so it's definitely not very famous. Then, if you knew about it, you'd probably guess it's in some kind of very fancy fortress, so you'd have to spend years of your life coming up with a plan and getting everything. And then, you'd search every single highly guarded place, finding absolutely nothing. Why? Because, guess what, the diamond isn't guarded at all! So, in short, the diamond was put in a very stealable place because, well, who'd expect it to be there?