A diamond is valuable, though it lie on a dunghill. 
Related proverbs
Similar proverbs in another language
German
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Gold glänzt auch im Schmutz.
Gold shines even in the dirt.
Czech
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Dobrý kůň i v maštali kupce najde.
A good horse can find a buyer even in a stable.
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Zlato zlatníka najde.
Gold will find the goldsmith.