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.