Looking for a word that means the opposite of 'gilded'

I'm looking for the opposite of gilded when used as:

having a pleasing or showy appearance that conceals something of little worth.

Most of the antonyms listed from different sources mean 'dull' or 'boring'. I'm looking for a word that literally means 'mundane on the outside but with a heart of gold'. Does such a word exist?

"Diamond-in-the-rough" is a literal expression that is also used figuratively for things of value with unassuming exteriors.

The earliest usage I found of this expression is in the Pennsylvania Constitution Convention (John Agg) Volume 5 - 1938:

"He may be looked upon as the diamond in the rough, which may, when properly polished, at some future day, become a bright and shining ornament and develope its inherent lustre."

As John Lawler points out, several amusing combinations of negations exist, although your qualification “mundane on the outside but with a heart of gold” limits possibilities somewhat. Here are a few words and phrases to consider.
• unpretentious, “simple, humble, not pretentious, plain”
• down-to-earth, “practical; realistic; pragmatic”
• salt of the earth, “A most worthy person ... A decent, dependable, unpretentious person”
• solid, “Large, massive ... Strong or unyielding ... Excellent, of high quality”
• earnest, “Serious in speech or action; eager; urgent; importunate; pressing; instant. ... Ardent in the pursuit of an object; eager to obtain or do; zealous with sincerity; with hearty endeavour; heartfelt; fervent; hearty ... Possessing or characterised by seriousness; strongly bent; intent ... Strenuous; diligent”

understated - represented less strongly or strikingly than the facts would bear out

It's not as metaphorical as 'gilded', but is semantically opposite in that the external appearances are not as good as what's behind.