Sergio (carbonado)
{{short description|Largest known rough diamond}}
File:PSM V69 D278 The largest carbon ever found.png engraving of the Sergio]]
The Sergio (Portuguese: Carbonado do Sérgio) was the largest carbonado and the largest rough diamond ever dug up on earth.{{cite web | last=William |first=Stephen E. |title=Carbonado Diamond: A Review of Properties and Origin |url=https://www.gia.edu/gems-gemology/summer-2017-carbonado-diamond |date=Summer 2017|accessdate=4 April 2018|publisher=Gemological Institute of America}} It was known to weigh {{convert|3167|carat|g ozt}}. But see the update below in which the weight is revised to 3,245 carats.{{Cite book |last=Farges |first=Francois |title=The Ultimate Secrets of a Cursed Diamond |publisher=self-edition |year=2025 |isbn=9782959797019 |edition=2nd |location=Paris |publication-date=2025 |pages=391 |language=en |trans-title=(translated from "Les ultimes confessions d'un diamant maudit")}} It was found above ground in Lençóis (State of Bahia, Brazil) in 1895 by Sérgio Borges de Carvalho.
The Sergio was first sold for $16,000 and later for {{Inflation|US|25,000|1895|fmt=eq|orig=yes|cursign=$}} to Joalheria Kahn and Co. and shipped to G. Kahn in Paris, who sold it to I. K. Gulland of London in September 1895 for {{Inflation|UK|6,400|1895|fmt=eq|orig=yes|cursign=£}}. It was then broken up into small {{convert|3-6|carat|g oz|adj=on||}} pieces as industrial diamond drills.{{cite web |last=Herold |first=Marc W. |date=April 2013 |title=The Black Diamonds of Bahia (Carbonados) and the Building of Euro-America: A Half-century Supply Monopoly (1880s-1930s) |url=https://commoditiesofempire.blogs.sas.ac.uk/files/2016/03/WP21.pdf |accessdate=4 April 2018 |publisher=University of New Hampshire |page=12}}
The precise circumstances surrounding Sergio's discovery, his export to Paris, and then to London, have been rediscovered and extensively corrected (as various previous publications, including, contain certain historical or scientific errors as in ).
Like other carbonados, the Sergio is believed to be of meteoritic origin.{{cite web|title=Carbonado - A possible relic from Uranus or Neptune|url=http://www.meteoritestudies.com/protected_CARBONAD.HTM|publisher=meteoritestudies.com|accessdate=15 February 2013|archive-date=30 October 2019|archive-url=https://web.archive.org/web/20191030180440/http://www.meteoritestudies.com/protected_CARBONAD.HTM|url-status=dead}}{{cite EB1911 |first=Frederick William |last=Rudler |wstitle=Carbonado |volume=5 |page=307}}[http://shell.cas.usf.edu/~juster/GLY4921/carbonado%20diamond%20paper.pdf G.J.H. McCall, "The carbonado diamond conundrum"] {{webarchive|url=https://web.archive.org/web/20140221111337/http://shell.cas.usf.edu/~juster/GLY4921/carbonado%20diamond%20paper.pdf |date=2014-02-21 }} However, most recent publications ([http://geosphere.gsapubs.org/content/early/2013/08/14/GES00908.1.full.pdf R.A. Ketcham, "New textural evidence on the origin of carbonado diamond: An example of 3-D petrography using X-ray computed tomography" Geosphere, GES00908.1, first published on August 14, 2013]{{Dead url|date=June 2024}} and many others summarised in ) have confirmed that this cosmological hypothesis is becoming less and less credible, as various teams have measured clearly terrestrial characteristics in various carbonados, including possible signatures isotopic {{Cite journal |last=Afanasiev |first=Valentin |date=2025 |title=About the Origin of Carbonado |journal=Minerals |volume=14 |issue=9 |pages=927–942 |doi=10.3390/min14090927|doi-access=free }} of terrestrial biological origin (via the 13C isotope and many other arguments).
Between 2023 and 2024, two historical casts (December 1895) of Sergio were found at the Natural History Museum in London,{{Cite journal |last=Hansen |first=Robin F. |date=2024 |title=Part 1: The Sergio: An Exploration of the World's Largest Carbonado. |journal=The Australian Gemmologist |volume=28 |pages=268–276}}{{Cite journal |last=Hansen |first=Robin F. |date=2024 |title=Part 2: The Life and Times of the World's Largest Carbonado |journal=The Australian Gemmologist |volume=28 |pages=308–318}} and an older one (September 1895){{Cite web |title=«L'histoire de "Sergio", le plus gros diamant jamais découvert» (in French) |url=https://www.mnhn.fr/fr/l-histoire-de-sergio-le-plus-gros-diamant-jamais-decouvert}} at the French National Museum of Natural History (acronym in French : MNHN) in Paris {{Cite journal |last=Farges |first=Francois |date=2025 |title=Nouvelles découvertes autour du Sergio, le plus gros diamant (noir) connu : 3245 carats (in French) |journal=Revue de gemmologie AFG |volume=223 |pages=16–23}} which cast had been made by the French chemist and Nobel Prize winner, Henri Moissan.
A photograph of Sergio before its destruction was even found at the French MNHN in 2025: it was originally published in 1913 in a book by J. Escard without its name ‘Sergio’, a name that was coined much later by the Gemological Institute of America around 1955. An image analysis using hierarchical clustering based on different linkage methods (WARD, SSIM) proves that the best-known photograph of this carbonado (1906, published above in Popular Science Monthly) is probably the photograph taken around 1900 of a fourth moulding then kept (not yet found) at the Instituto Geográfico e Histórico da Bahia (IGHB). This photograph shows an object that has all the characteristics of the cast found in Paris and donated by Moissan to the IGHB.
Finally, Sergio's weight was revised in 2025 because it was given in 1895 in old Brazilian carats and had not been converted to modern metric carats: thus, the published weight of 3,167 carats (actually old carats or karats) has been corrected to 3,245 modern metric carats, as confirmed by the analysis (scanner) of the Paris cast at the MNHN as well as writings of Henri Moissan himself in 1895.
A fifth cast of the Sergio was 3D printed from polylactic acid and donated in 2025 by MNHN Prof. Francois Farges to the Sociedade União dos Mineiros (SUM, Mining Union Society) in Lençóis, where it has been on display ever since.