(ShareCast News) - Gem Diamonds sold an "exceptional" 357-carat white diamond for $19.3m on tender in Antwerp last week.Chief executive officer Clifford Elphick said the Letseng mine produced two "remarkable" diamonds of over 300 carat during the year to date.He pointed to the 314 carat diamond, which was recovered in May 2015 and sold into a partnership arrangement in June 2015, and the 357 carat diamond sold last week."Both of these diamonds achieved top prices despite current market conditions, providing further evidence of the price resilience of Letseng's large top quality diamonds," said Elphick.At 1526 BST, Gem Diamonds shares were up 1.8% at 127.25p.