Miranda Hollingshead, 27, was visiting Arkansas' Crater of Diamonds State Park with her family when she made the discovery.
Renowned jeweller Harry Winston was the buyer, Christie’s said.
A flawless 100-carat diamond ring has sold for £14.8 million at auction in New York.
A 13.22 carat diamond known as "The Blue" has fetched over £13 million at an auction at Christie's auction house in Geneva, according to AFP.
The sale comes a day after Sotheby's sold a 100-carat flawless yellow diamond for £9.7 million.
US jeweller Harry Winston bought the blue diamond, described by Christie's as the largest fancy vivid blue diamond in the world.
More: Large flawless blue diamond could reach $25 million