Gem Diamonds unveiled two significant diamond recoveries from its Letseng mine in Lesotho, southern Africa.
Shares in the London-listed miner rose more than three per cent following the news.
The firm's Letseng mine is renowned for producing large, top colour exceptional white diamonds, and it is the highest dollar per carat kimberlite diamond mine in the world.
Gem Diamonds unearthed a high-quality 104.73 carat, D-colour Type IIa diamond and a 151.52 carat Type I yellow diamond from the mine.
In a note to investors, RBC Capital Markets said the recovery of two 100-plus carat stones should be a positive for both sentiment and cash flow. The white diamond could fetch about $4m to $5m (£3m to £4m) auction, RBC said, while the yellow diamond, which is rarer, could sell for a similar or higher value.
Gem Diamonds acquired Letseng in 2006, and since then it has produced four of the 20 largest gem-quality white diamonds ever recorded. In February it was forced to mothball a mine in Botswana due to low diamond prices.