(Bloomberg) — Petra Diamonds Ltd. sold a 39.3-carat blue gem for more than $40 million, making it one of the most expensive rough diamonds ever.

The small miner sold the exceptional Type IIb blue diamond to a joint venture between top producer De Beers and Diacore, a trading company owned by the billionaire Steinmetz family, it said Monday. The stone fetched just over $1 million per carat and is the most expensive gem Petra has ever sold.

Petra found the diamond at the Cullinan mine in South Africa in April. The mine, once owned by De Beers, is famous for both large and blue stones and was where world’s biggest diamond was found in 1905. Blue stones are among the most rare and valuable.

The sale is good news for Petra, which was forced to restructure its debt last year, when the Covid-19 crisis brought the industry to a standstill at a time when the company was already facing a mountain of debt and falling diamond prices. The shares, which were once worth more than $1.5 billion, closed up 1.1% on Monday.

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