An 89.75 carat yellow diamond was recovered during the first few days of processing kimberlite material from the south-east zone, providing evidence that the project is likely to be a source of large diamonds.
Last month, a 25ct stone was recovered from Mothae's neck zone during the first two days of trial processing through the bulk sampling plant after recent efficiency and security upgrades.
Lucapa and its 30% partner, the government of Lesotho, are hoping to expand and improve Mothae's current 1 million carat JORC resource by processing areas of the kimberlite pipe that weren't previously included in the resource or where they believes there has been insufficient historical sampling.
The plan is to process three 5,000 tonne bulk samples of kimberlite from the south-east, north and neck zones.
Lucapa said the construction of the new 150 tonne per hour commercial diamond plant was on schedule for commissioning in the second half of the year.
Managing director Stephen Wetherall said the 89.75ct recovery was encouraging, as the south-east zone's kimberlite pipe had had very limited historical testing.
"This underlines our belief in Mothae. It also vindicates Lucapa's decision to commit resources to the additional bulk sampling programme," he said.
Lucapa secured a US$15 million finance facility to develop Mothae last year, which has an estimated total indicated and inferred resource of 38.96 million tonnes at a grade of 2.7 carats per 100 tonnes, containing 1.04Mct of diamonds at an average revenue of $1,063 per carat.
Lucapa's shares were trading at A$0.28 (US$0.21) Friday, up 7.84%