Most Expensive Diamond in the World

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If we measure by price per carat, the story gets wild. The “most expensive per carat” title belongs to the Blue Moon of Josephine, a 12.03-carat fancy vivid blue diamond that sold for $48.4 million in 2015. That’s $4 million per carat. For perspective, the average diamond wedding ring costs about $5,000. This single diamond is worth 10,000 of those rings. It’s like buying a Ferrari and then paying for it in gold bullion.

Why so much? Blue diamonds are even rarer than pink ones. They get their color from trace amounts of boron, which sounds like something from a high school chemistry class but actually makes a gem worth more than a house in Beverly Hills. The Blue Moon was bought by a Hong Kong billionaire who named it after his daughter. I hope she’s careful not to lose it under the couch cushions.

The Rumor Mill: The “Most Expensive” That Never Sold

Now, let’s get into the rumors. Some say the Cullinan Diamond (the largest rough diamond ever found, at 3,106 carats) is worth billions. But it’s a rough stone that was cut into nine major gems, including the massive Cullinan I and Cullinan II, which are now part of the British Crown Jewels. You can’t buy them—they’re guarded by the Queen’s corgis and some very serious security guards.

The Most Expensive Diamonds in the World (2026 List) - BlueStone Blog

Then there’s the Sancy Diamond, a 55-carat pale yellow beauty with a history of being owned by kings, and the Centenary Diamond, a 273-carat monster discovered in the De Beers mine in 1986. All of these are “priceless” because no one will ever sell them. So, for the sake of this article, the Pink Star is our champion. But honestly, the whole list is like a hierarchy of absurdity—diamonds so expensive they make you laugh and cry at the same time.