private jewelry

i had this one diamond that i like the most. i talk to this diamond everyday. i dont know how did i get my hands on this one diamond. but one day, the rightful owner of the diamond show up, asking me nicely to return what is rightfully his.

if ur in my place, what would you do?
i had a few things in mind.

he lost the thing, and out of a sudden, i came into my possession. whos fault it is?
eventho he did leave a mark there proving that diamond is his, he just cudnt just show up want it back.
and the diamond isnt really mine to begin with.
what will happen if i keep what isnt mine?
and what will happen if let go things i like the most?