There is an expression, "like putting lipstick on a pig", which means (in context) that if you take something obviously bad, no matter how many cosmetic changes you make to it, it will remain obviously bad.
Is there an expression that means the opposite? That is, if there is something obviously good, no amount of minor blemishes will take away from it.
Best Answer
Maybe "diamonds in the rough" is close to what you mean.
Uncut diamonds don't look so good. But they're worth diamonds either way.
There are different variations of it.
Wiktionary example: