The agency was first called upon last year for the sale of the 603-carat Lesotho Promise diamond. Merlin MD David Simonsen handled media coverage of the Lesotho, the biggest diamond discovered for 13 years. It sold for more than £6.5m.
Last week Merlin oversaw Gem Diamonds' IPO, which saw the company valued at over £550m.
An agency team of four reports to Gem Diamonds chief financial officer Kevin Burford.
The Lesotho Promise is the 15th largest diamond ever found. It was followed this month by a 215-carat diamond, the sale of which Merlin will also oversee comms-wise.
The stone was recovered at Letšeng Diamond Mine in southern Africa, a mine renowned for the quality and size of its diamonds. The mine is owned by Gem Diamond Mining Company of Africa, with its 30 per cent partner the Government of the Kingdom of Lesotho.
Gem Diamonds said it would use money generated from the sale to develop projects in the Central African Republic and elsewhere.
The industry is in the spotlight after the release of Hollywood film Blood Diamond, starring Leonardo Di Caprio as a South African smuggler who sells ‘conflict diamonds'.