Opposite of lipstick on a pig

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.

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:

The auto mechanic is a diamond in the rough, tough-talking, but honest, even generous.

gild the lily

Fig. to add ornament or decoration to something that is pleasing in its original state; to attempt to improve something that is already fine the way it is. (Often refers to flattery or exaggeration.) Your house has lovely brickwork. Don't paint it. That would be gilding the lily. Oh, Sally. You're beautiful the way you are. You don't need makeup. You would be gilding the lily.

http://idioms.thefreedictionary.com/gild+the+lily