Call it "peak diamonds". De Beers, the South African miner that is the world's biggest producer, says that the world's sources of the gems are depleting and are not being replaced fast enough to meet long-term demand. Many analysts and traders agree.

The argument sounds much like the controversial "peak oil" theory, which in its most extreme version says the world is already running out of oil. With diamonds, analysts differ over the exact timing of when peak output will be reached.

In an industry built around myths the idea is appealing, because it draws more attention to the rarity of the sparkling rocks. The miners - and De Beers in particular - have an interest in promoting this view, even if some commentators say it is not necessarily true.