A 198-carat white diamond of excellent quality has been discovered in Lesotho, mining firm Gem Diamonds announced Tuesday.

The stone was found in the Letseng mine in the kingdom's northern Maluti mountains in July, the firm said in a statement.

"This exceptional white, high-quality diamond displays no florescence and is expected to achieve an exceptional price when sold this year," the company said.

One of the highest diamond mines in the world at 3,100 metres (10,000 feet), Letseng has delivered five of the top 20 rough white gems ever recorded, according to the firm.

In 2011 Gem Diamonds discovered a 550-carat white diamond in the tiny and mountainous kingdom, which is completely surrounded by South Africa.

In June this year, Petra Diamonds Cullinan said it had found a rare blue 122.52-carat gem in a South African mine where the world's largest diamond was unearthed in 1905.

In February Sierra Leone announced the discovery of a 153.44-carat stone worth $6.2 million, declaring it one of the most precious finds of the past decade.

Gem Diamonds owns a 70-percent stake in the Lesotho mine, while the government holds the remaining 30 percent.