A 10 carat blue diamond valued at 20 million is set to headline a Geneva auction in May. Named The Mediterranean Blue, it weighs 10.03 carats and carries a VS2, a clarity grade with small inclusions under 10x magnification. Sotheby’s has listed the stone in its online listing for the Geneva sale for viewing by registered bidders.
What the latest sales say about demand
Lucapa Diamond Company says two colorless Type IIa diamonds from Angola sold for 3.5 million at a March tender. The company disclosed the result in an announcement on March 14, 2025.
“The first tender result for 2025 has been very encouraging, with strong prices received for both stones.” said Alex Kidman, managing director and CEO of Lucapa Diamond Company.
Both Lulo stones weighed 93 carats and 78 carats, and together averaged 20,400 per carat. Type IIa, diamonds with no measurable nitrogen impurities, are rare in nature and can appear exceptionally transparent.
What makes a blue diamond blue
The Mediterranean Blue owes its color to trace boron atoms within the diamond lattice. That point is supported by a study on boron bearing blue diamonds.
Some blue diamonds likely formed deep in the lower mantle, where pressure is extreme and water bearing minerals can exist. That deep origin helps explain their scarcity and high prices in many auction cycles.
On the color scale for colored stones, ‘Fancy Vivid’ marks the strongest saturation grade used by labs worldwide and the top tier of colored diamond color intensity, which is a description that tells you the hue is rich and stable.
Sotheby’s notes that the stone is Type IIb, a boron bearing class that can conduct electricity at room temperature. Type IIb, a rare category with boron atoms in the lattice, is distinct from Type IIa seen in the Lulo pair.
Why type IIa stones stand out
Gemologists classify diamonds by the impurities they carry, and Type IIa stones sit near the top for purity. A GIA primer notes that Type IIa diamonds lack measurable nitrogen and often appear colorless.
Lucapa recovered the two stones at the Lulo Alluvial Mine in northeastern Angola. Alluvial, deposited by rivers and floods over time, describes diamond rich gravels that concentrate heavy minerals in natural traps along bends and bedrock pockets.
Rough diamonds start life in volcanic rocks called kimberlites before erosion moves them into rivers. Kimberlite, an igneous rock that brings deep minerals to the surface, is the primary source of most natural rough.
Price talk often comes down to weight and rarity, measured as dollars per carat. Per carat, the price divided by the stone’s weight, helps buyers compare stones of different sizes.
How tenders work and why Angola matters
Tenders are different from fixed price or negotiated sales, because sellers display parcels and accept sealed bids on a set date. This format can reduce guesswork and reveal what the market is willing to pay for a specific stone.
Angola’s state marketer, Sodiam E.P., runs regular tenders that attract buyers from cutting centers across the world. Competitive tendering has helped place Lulo’s large stones in front of buyers who specialize in high end cutting.
Large stones often sell as single lots, because each has unique cutting plans and risks. Buyers model likely shapes and yields, then bid what those future polished goods could be worth after accounting for losses.
Viewings are typically in person, with strict security and detailed measurement of each stone, and calibrated lighting for color checks. Sellers sometimes split production into parcels for size ranges and hold separate single stone events for exceptional material.
Where prices might go next
Top colored diamonds can defy broader headwinds, and the 20 million valuation shows that appetite for rarities persists. High profile lots set expectations for other large stones and can lift confidence beyond the auction room.
At the same time, miners have trimmed production to balance inventories after a soft 2024. De Beers reported a sharp cut in the second quarter of 2025. That move can support prices if demand firms.
Lucapa’s result at Lulo fits that cautious improvement story. The company’s average price for the two stones was 20,400 per carat, with the 78 carat stone achieving the higher price per carat.
Auction outcomes can swing month to month, especially for colored diamonds. Currency shifts and interest rates can nudge bids, while careful buyers value crystal health and provenance.
What this means for students of Earth
Blue diamonds sit in a different chemical class from the Type IIa Lulo stones, but both categories reveal something about Earth. Blue stones can carry boron from ancient oceanic plates, while Type IIa stones whisper of unusual clarity in the mantle’s chemistry.
For young scientists, the link between subducting slabs and deep diamond growth is a reminder that gems are part of the rock cycle. The science here connects plate tectonics, mineral physics, and the trace elements that color carbon crystals.
Cutting rooms will watch the Geneva result and the next rounds of tenders. They will also watch consumer traffic in key markets as the year unfolds.
If high end demand holds, cutters may shift capacity back toward big single stones. If retail remains uneven, miners will likely keep adjusting supply to avoid a glut of mid range goods.
