Petra’s 42-carat Cullinan blue diamond: value and mine-planning notes for engineers
Reviewed by Tom Sullivan
First reported on MINING.com
30 Second Briefing
A 41.82-carat type IIb blue diamond has been recovered from Petra Diamonds’ Cullinan mine in South Africa, with independent analyst Paul Zimnisky calling it likely the largest high-quality fancy-blue diamond found in modern history and estimating a value in the “tens-of-millions” of dollars, potentially up to $40 million. The find extends Cullinan’s record of ultra-rare blue stones, following the 2022 De Beers Cullinan Blue ($57.5m) and the 10-carat Mediterranean Blue cut from a 31.94-carat rough sold for $21.5m in 2025. The discovery offers a timely boost for Petra and a natural-diamond market under pressure from weak demand and lab-grown competition.
Technical Brief
- Rough stone is type IIb, with boron impurities giving the blue hue via warm-tone absorption.
- Type IIb diamonds constitute under 0.1% of natural diamonds, underscoring Cullinan’s unusual geochemical environment.
- Specialists are currently assessing optimal cutting strategy and market timing, indicating complex value-optimisation decisions ahead.
- Cullinan mine has operated since 1902 and is planned to remain productive into the 2040s.
- Historic Cullinan blue diamonds include a 24.18‑carat intense blue sold for $25 million in 2016.
- The 14.62‑carat Oppenheimer Blue achieved 56.8 million Swiss francs (~$71.3 million) at Christie’s Geneva.
- Mediterranean Blue, cut to 10 carats from 31.94‑carat rough in 2023, sold in Geneva in May 2025.
Our Take
This is one of only 21 diamond‑keyword pieces in our database, and alongside the 2026 Taylor Swift ring article it reinforces that natural diamonds – especially from legacy assets like Cullinan in South Africa – are still commanding narrative space despite the broader shift of mining coverage towards critical minerals and base metals.
Petra Diamonds’ ability to recover a 41.82‑carat type IIb stone from Cullinan, which has been in production since 1902, underlines the strategic value of long‑life, fully permitted African kimberlite operations at a time when new greenfield diamond projects face much tougher ESG and financing scrutiny.
The $360 million Colossus rare earths project in Minas Gerais, mentioned alongside Petra’s blue diamond, highlights how capital is now split between high‑margin legacy luxury commodities (diamonds) and new energy‑transition inputs (rare earths), a pattern that suggests diversified investors will benchmark such one‑off diamond windfalls against multi‑decade critical mineral capex opportunities.
Prepared by collating external sources, AI-assisted tools, and Geomechanics.io’s proprietary mining database, then reviewed for technical accuracy & edited by our geotechnical team.
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