different as chalk and cheese
Definition from Wiktionary, the free dictionary
Attested in variant form 1393. The surface of some cheeses, particularly unaged ones, looks similar to chalk – white, crumbling – but in substance cheese is much softer than chalk.
- (UK, simile) Two things that are different, particularly two things that are superficially alike but very different in substance.
- "chalk and cheese", World Wide Words, Michael Quinion