What it means
A person who’s a bit uncouth, blunt, or scruffy on the surface, but solid gold underneath. You use it when someone’s manners need work, yet they’re honest, loyal, and properly kind when it counts. It’s just the gem metaphor: not polished, still valuable. Often said with a fond, forgiving tone.
Usage examples
"He’s a rough diamond, that one, mouth like a docker but he’d give you his last tenner and walk you home."
"He swears like a sailor but he is a rough diamond, heart of gold."
"The new apprentice is a rough diamond, raw but full of promise."
"She’s a rough diamond, no question. Bit sharp round the edges, but she turned up at 2 a.m. with jump leads and a coffee."
"Don’t let the barking put you off, he’s a rough diamond. Says everything too blunt, then quietly sorts everyone out."
Where it comes from
It comes from the literal idea of an uncut diamond, valuable stuff before anyone’s polished it up. English was already using rough diamond for people by the 1800s, especially for someone blunt, awkward, or coarse on the outside but solid and decent underneath. The image stuck because it lands instantly.
