And if you can't find all the pieces, what then? And even if you did, assuming a human skull is an "object" (which I dispute), there will certainly be more than a single tear or break. So what, is each pair of puzzle pieces now an object you can cast mend on?
I don't think that matches the intention of the spell at all.
Although CSI Faerun would be kinda fun, so maybe I'd allow it.
Mending fixes a single tear or break in an object. No way should that work on a head that's been splattered halfway across the Sword Coast.
And if you can't find all the pieces, what then? And even if you did, assuming a human skull is an "object" (which I dispute), there will certainly be more than a single tear or break. So what, is each pair of puzzle pieces now an object you can cast mend on?
I don't think that matches the intention of the spell at all.
Although CSI Faerun would be kinda fun, so maybe I'd allow it.