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The latest find, a 357ct light brown high-quality diamond, brought the company's tally for the year to a new record, with seven recovered in 2017.
The largest diamond found this year was a 910ct D colour type 2a diamond called the Lesotho Legend, which was the second largest recovered in the past century and sold for US$40 million in March.
Gem's shares jumped 2.17% Tuesday to £1.175 (US$1.55).