A person or thing with exceptional qualities or potential that is not immediately apparent due to a rough, unpolished, or unpromising exterior. Someone with hidden talent or value that needs development to shine.
The phrase originates from the literal process of diamond mining and cutting, where raw diamonds look like ordinary rocks until they are cut and polished. The metaphorical use dates back to the 17th century, with John Fletcher using a similar expression in his 1624 play "A Wife for a Month."
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