Gem recovered 126,875ct during the year, up 13% from 2017, from 6.5 million tonnes of treated ore, steady, at a grade of 1.94 carats per hundred tonnes, 11% higher than the previous year.
The company's sales of rough diamonds rose 17% year-on-year in 2018 to 125,111ct at an achieved price of US$2,131 per carat, up 10%, with the total value increasing 29% to $266.6 million.
It said it sold 44 diamonds for more than $1 million each, generating revenue of $137.2 million.
Gem Diamonds as a group had $50.8 million of cash on hand at the end of December and a net cash position of $17.5 million.
It drew down $33.3 million of available facilities and had undrawn and available facilities of $57.8 million at December 31.
During the December quarter, Gem said Letšeng paid $17.4 million in dividends, resulting in a net cash flow of $11 million to Gem Diamonds and a cash outflow from the group for withholding taxes of $1.2 million and payment of the Government of Lesotho's share of dividend of $5.2 million.
CEO Clifford Elphick said production in 2018 included a record number of diamonds greater than 20ct recovered in a single year.
"The revised mine plan results in a significant reduction in Letšeng's overall stripping ratio due to steeper slope angles, which is expected to result in a significantly improved NPV of the mine," he said.
"Good progress has been made on the business transformation process which remains on track to achieve its cumulative four-year target of $100 million, with already implemented initiatives expected to deliver $60 million over the four-year target period."
Gem's shares have risen 15.2% in the past year to the current £1.035 (US$1.34) per share, although they've fallen 7.6% since the start of 2019.