Twelve miles off Namibia’s arid southern coastline, 150 metres below rolling ocean waves, diamond miners are hard at work securing a future for the practice of romancing by stone.

With the precious gems expected to run out on land in as little as 15 years, diamond company De Beers is building up a naval fleets to protect its interests.

Namdeb, De Beers’ 50:50 operation with the Namibian government, estimates that 95 per cent of its diamonds will in future come from the sea bed off Africa’s southwest coast and the marine gems are already the fetching the highest prices from all of its seven mines.