Hard as a Rock

The phrase ‘Hard as a Rock’ means very hard.

Example of Use: “This cake is as hard as a rock!”

Interesting fact

Rocks and stones are among the hardest surfaces known to mankind, and though items described by the idiom 'hard as a rock' might not literally be as hard as a rock, they’re typically much harder than the user would like them to be. No one is certain when this term was first used to figuratively describe something, but what is certain is that it’s an idiom that’s not difficult to understand.