the.com/diamond mine

means A business or opportunity so lucrative in reputation that it draws investors, even though it often swallows more money than it yields.

from The literal diamond mine — a pit dug to extract rough diamonds — became a byword for guaranteed riches sometime after the 19th-century diamond rushes in places like South Africa, where a lucky few struck it fabulously rich while most diggers struck mud and debt. The phrase 'it's a diamond mine' (meaning a sure source of wealth) rode on that glittering myth, while the grimmer truth — that most mines, like most 'can't-miss' ventures, mainly produce dirt and disappointment — got quietly dug in underneath it.