Idiom / phrase for a treasure in a ruin

How to describe some thing valuable that could be gained from an unexpected place? I heard a Persian Idiom that could be appropriate for this case:

a treasure in a ruin.

Is there any equivalent idiom or phrase for that?

Thanks to @Mazura for the thorough research (+1), in Persian literature The man of God is a treasure in a ruin means that a pious/virtuous human is as valuable as a treasure and very reliable in difficulties. So, this is not the case where we use the idiom , however we may say (s)he is as worthy as a treasure in a ruin.

As another (less) probable source for the idiom, below is a poem from another great Persian poet "Sa'di" (alongside with "Ferdowsi, the Great", "Rumi" and "Hafez"):

If livelihood were increased by knowledge /None would be more needy than the ignorant.

Nevertheless the ignorant receive a livelihood/At which the learned stand aghast.

The luck of wealth consists not in skill/ But only in the aid of heaven.

It happens in the world that many / Silly men are honoured and sages despised.

If an alchemist has died in grief and misery/ A fool discovered a treasure amidst ruins.

(The Gulistan of Sa'di from http://classics.mit.edu/Sadi/gulistan.2.i.html)

There is the idiom of a diamond in the rough:

someone or something whose good qualities are hidden This film is one of those diamonds in the rough, a wonderful gem that almost no one has noticed.

Etymology: based on the idea that you cannot see the beauty of a diamond (jewel) when it is rough (not yet cut and filled with brightness)
_{The Free Dictionary by Farlex}

An uncut diamond doesn't look much different from ordinary quartz, as illustrated by the picture below of the 253-carat Oppenheimer Diamond.

A lily among thorns.

Something positive that stands out from the negative {snakeeyedbarbie at Yahoo! Answers}

As the lily among thorns, so is my love among the daughters

Song of Solomon 2:2; KJV

A more literal version would be a treasure trove.

Noun

1. treasure trove - treasure of unknown ownership found hidden (usually in the earth)
   synonym: trove:
   related: hoarded wealth, treasure - accumulated wealth in the form of money or jewels etc.; "the pirates hid their treasure on a small island in the West Indies"

2. treasure trove - any collection of valuables that is discovered
   "her book was a treasure trove of new ideas"; "mother's attic was a treasure trove when we were looking for antiques"
   related: aggregation, collection, accumulation, assemblage - several things grouped together or considered as a whole

Reference:
http://www.thefreedictionary.com/treasure+trove

It's funny, the common phrase, today, I think which is honestly closest to just what you mean is simply:

"a real find"

Also, "What a great find." You're talking about the case where you, for example literally, purchase a great thing at a garage sale.