Rough tender revenues for the January-June period were US$169.2 million, 43% higher than the previous half-year period.
It sold 61,696 carats, an 8% increase from the July-December 2017 period, at an average price of $2,742 per carat, up 33%.
A revenue of $99.1 million was generated from the sale of 25 diamonds alone, which all sold for more than $1 million each.
The high revenues are no surprise, with one of the diamonds, the Lesotho Legend, a high-quality 910ct, D colour, type IIa diamond, sold for $40 million, or $43,912/ct in March, and nine other stones over 100ct recovered during the half-year.
The highest prices achieved were $62,433/ct for a 2.26ct pink diamond and $56,028/ct for a 66.27ct type IIa white diamond.
Gem recovered 61,596ct during the six-month period, steady from the previous half-year period, although the grade increased to 2.06 carats per hundred tonnes, from 1.88cpht in the July-December 2017 period.
It stripped 13.5 million tonnes of waste in the January-June period, down 8% from the previous six-month period in line with the mine plan, with the volume of ore treated also down 8% to 3Mt.
Gem said Letšeng paid dividends of $51.8 million during the half-year, resulting in a net cash flow of $32.6 million to Gem and a cash outflow from the group as a result of withholding taxes of $3.7 million and payment of the Government of Lesotho's share of dividend of $15.5 million.
At the end of June, the group had $70.5 million of cash on hand, of which $58.1 million was attributable to Gem, and a net cash position of $29.8 million, $22.1 million attributable to Gem.
It said it had drawn down $40.7 million of available facilities, leaving undrawn and available facilities of $38.2 million.
Gem CEO Clifford Elphick said the total 11 over-100ct diamonds recovered so far in 2018 was the company's highest number in a single year.
"Greatly improved recoveries combined with the discovery and subsequent sale of the Lesotho Legend has generated a strong financial performance for the first half," he said, adding the market for Letšeng's high-quality diamonds had remained robust.
Gem also noted it had received initial non-binding offers for its mothballed Ghaghoo mine.
The company's shares reacted well, rising 2.6% Friday to £1.185 (US$1.54).