The diamond is the second weighing more than 100cts the company has recovered this year, after it produced a 127ct stone earlier this month.
The company said it would put both diamonds up for tender in March, along with other large stones it had recovered at the mine.
The 240ct stone was from the M/PK(S) unit in the south lobe of Karowe.
CEO Eira Thomas said the South Lobe was providing continuing confirmation that large, high-quality gem diamonds were being recovered throughout the kimberlite, in all lobes and phases on a regular basis.
Since mining started at Karowe in 2012, the company has recovered 54 diamonds weighing more than 200cts each.
Lucara, which pays a progressive quarterly C2.5c dividend, has seen its equity depreciate 36% over the past 12 months to $1.59 on Monday, capitalising it at $630.5 million.