The 2026 Cullinan blue diamond

What did Petra find at Cullinan in December 2025?

A 41.82-carat rough blue diamond, classed as Type IIb, which Petra described as exceptional in colour and clarity.

The find was reported by Mining Weekly on 13 January 2026, from a Petra media release. At that point Petra was still analysing the stone and choosing how to sell it. No polished grade has been published, and none can be until the stone is cut.

The timing was good news in a bad year. Petra was fighting weak prices for small rough diamonds, and a big blue is the opposite of a small rough diamond. One stone like this can change a quarter's revenue.

How was the 41.82-carat blue diamond sold?

Through a competitive tender, for a price Petra keeps confidential, with a profit share on the polished stone and a commitment to cut it in South Africa.

Petra's Q3 FY2026 sales results of 1 April 2026 give the structure. The buyer's name is confidential. Petra received an initial payment linked to the value of the rough, booked in that quarter. It keeps "a significant economic interest" in the profit when the polished stone is sold, which Petra says could take about 12 months.

The stone will be cut and polished in South Africa, which Petra presented as support for the local cutting industry. Joint CEOs Vivek Gadodia and Juan Kemp said they "look forward to seeing the ultimate outcome of this magnificent gem". Reading the timeline, a polished result could appear around the second quarter of 2027. That is our estimate, not Petra's date.

How does it compare with Cullinan's other blue diamonds?

It is a little heavier than the 39.34 ct blue of 2021, which sold rough for US$40.18m. Cullinan's largest recorded blue rough is the 122.52 ct stone of 2014.

The 2021 stone is the obvious comparison. Petra recovered the 39.34 ct Type IIb in April 2021 and sold it in July 2021 for US$40.18m to a partnership of De Beers and Diacore (Rapaport). According to GIA, which graded the result, that rough was cut into the 15.10 ct De Beers Cullinan Blue: Fancy Vivid blue, Internally Flawless. It sold at Sotheby's Hong Kong on 27 April 2022 for about US$57.5m.

Notice what that means. A cutter turned 39 carats of rough into 15 carats of polished, and the price rose by around US$17m. Weight is lost in cutting, and value is gained if the colour holds. That is why Petra kept a profit share this time.

Why does the Cullinan mine produce blue diamonds?

Boron. Type IIb diamonds contain traces of boron and almost no nitrogen, and the boron absorbs light in a way that makes them blue. Cullinan is the main source of them today.

GIA's 2018 study of natural blue diamonds explains it: uncompensated boron creates an absorption that gives the blue colour, and the mix of boron with no detectable nitrogen needs "extremely rare" conditions. Research cited there (Smith and others, 2018) puts these stones among the superdeep diamonds, formed more than 600 km down, far below most gem diamonds. The boron probably came from old ocean floor dragged into the mantle.

The amounts are tiny. GIA measured about 0.33 parts per million of boron in the De Beers Cullinan Blue. The same GIA study states that most Type IIb diamonds today come from the Cullinan mine, and Petra calls the mine the world's most important source of them. Our guide to the famous diamonds of Cullinan covers the colourless giants too.

Can you see the blue diamond on a Cullinan mine tour?

No. The stone was sold and is to be cut by its buyer in South Africa. The tour's display room shows replicas of famous diamonds, and the mine itself is the real thing.

Real rough of this value never goes on a public tour. What you get on the surface walking tour is the context: an introductory film, a display room with replicas of famous stones, a life-size model of an underground tunnel, the working shafts that hoist ore from 934 m, and the Big Hole lookout (Premier Diamond Tours). A jewellery shop comes last. Do not expect a blue in the window.

The tour does make the numbers feel real. Standing at the rim, you are looking at the pipe that produced the Cullinan Diamond, the 2021 blue and this one.

The film, the replica diamond room, the shafts and the Big Hole lookout, on a 2-hour walk at the working mine.

★ 4.6 · 2 hours · from $12

Explore the surface walking tour → Tour details

What happens next with the 41.82-carat blue?

It is to be cut and polished in South Africa. Petra will report the profit share when the polished stone sells, and it said it would give updates as appropriate.

We found no news of a polished stone, a name or a buyer between April and October 2026. One new factor is Petra itself: it started a strategic review on 1 October 2026 that could lead to a sale of its assets. Its share of the profit from this stone is one of those assets. That changes the paperwork, not the diamond.

When the cut stone appears, we will add its weight and grade here. Until then, treat any polished carat figure you see online as speculation.

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