In 1755, an earthquake of magnitude 8.5 -9.0 or greater struck in the Atlantic just west of Lisbon. Three separate quakes caused many of the great buildings in Lisbon to collapse to the ground. Shortly thereafter a set of three tsunamis crashed ashore and destroyed the shipping in the Tagus river. After the waters receded, fires from candles and furnaces in damaged churches and workshops broke out and spread uncontrollably throughout the city.
“One of the first structures to catch fire was the stately palace of the Marques de Lourical… it was a repository of countless treasures, including a collection of over two hundred paintings by such masters as Titian, Correggio, and Rubens, and a renowned library of eighteen thousand books, which contained a history composed by the Emperor Charles V in his own hand, a collection of preserved plants (a herbarium) once owned by King Matthias Hunyadi of Hungary, and a priceless assemblage of original manuscripts, maps, and charts from the Portuguese Age of Exploration…Prior to this tragedy, Portugal had been one of the maritime powers of the world; for centuries it had been harvesting the mineral wealth of Brazil and other colonies. The tragedy also destroyed all the public records of baptisms, births, burials, genealogy, the account books of merchants, treaties, contracts, rents, receipts, financial and economic legislation, and much of the paper money.
[at the Riverside Palace] “The fire burned all of the galleries, halls, rooms, antechambers, and offices of the palace, with all of its rich decorations and furniture covered in gold, silver, and rich jewels of inestimable value,” wrote Portal. (p. 155)
Gone forever were all the singular and extraordinary objects collected by the kings of Portugal over the centuries. Gone, too, was the Royal Library, “the most excellent in Europe” added Portal. The pride of the late Joao V, and indeed all of Portugal, the entire library and its seventy thousand volumes, reams of priceless manuscripts, including many of the original travel logs of Vasco da Gama and other Portuguese explorers, rare tapestries, oil paintings, and engravings were incinerated (though a few objects may have been pilfered by thieves and subsequently lost). In terms of cultural harm, the destruction of Portugal’s Biblioteca dos Reis (Library of the Kings) ranks as one of the great tragedies in the history of the West and can be likened to the burning of the Ancient Library of Alexandria. (p. 165)
“… the court in Lisbon was the richest in Europe in precious stones and it lost all of them, except those that the royal family had with them at the time of the disaster.” Indeed, “the two streets, where the richest goldsmiths and diamond setters lived, were those that suffered the most in the earthquake and the fire. In Goudar’s estimation,"two hundred diamond shops were completely buried under the ruins.”
Also “lost were priceless suits of armor,” wrote Father Portal, “especially those from the Royal Treasury belonging to his Majesty, jewels of incomparable value, diamonds, pearls, emeralds, [and] every kind of precious stone, [as well as] gold, silver, paintings, and statues.” (p. 272-3)
“As to diamonds, “11 to 12 million [cruzados’ worth of] diamonds” were lost in the India House alone… (about 1/3 of the official value of all the diamonds extracted from Brazil between 1740 and 1755.)
I've transcribed the text excerpts above from This Gulf of Fire: The Destruction of Lisbon, or Apocalypse in the Age of Science and Reason, by Mark Molesky.
See also this post from last month: The Library of John V of Portugal.