Its latest recovery is its eleventh large stone for the year, a 100.5ct top white colour type 2a diamond from its 70%-owned Letšeng mine in Lesotho.
Among the other 10 is the Lesotho Legend, an exceptional quality 910ct D colour type 2a diamond that is the second largest recovered in the past century. It was sold in March for US$40 million.
In 2017, Gem recovered seven over-100ct diamonds.
The company's shares were down 2% Friday to £1.145 (US$1.50).