There are two Japanese idioms I really like:

1. Kawara wa migaite mo tama ni naranu, no matter how much you polish a roof tile it won't become a gemstone.

2. Kawara mo migakeba nara tama to naru, if you polish it enough even a roof tile will become a gemstone.

I just like having those two contradictions.

I like:

Kaeru no tsura ni shonnbenn, カエルの面に小便, (Like) piss on a frogs face;

Basically the idiom "like water off a ducks back"