The relic is a ring of plaited rush (juncus balticus), without the thorns themselves — which had been removed and distributed individually as separate relics over the centuries (notable thorns are at Andria in Italy, at Stonyhurst in England, and elsewhere). The original was preserved at Constantinople from the early Christian centuries; in 1238, the impoverished Latin Emperor Baldwin II offered it to King St. Louis IX of France in exchange for substantial financial relief.
Louis met the relic, brought by Dominican friars, at Villeneuve-l'Archevêque in 1239 and carried it himself, barefoot, into Paris. He built the spectacular Sainte-Chapelle on the Île de la Cité (1241–48) specifically to house it. After the French Revolution it passed to the Bibliothèque Nationale and then to the Cathedral of Notre-Dame de Paris. On April 15, 2019, in the fire that destroyed the spire and roof of Notre-Dame, the Crown of Thorns was rescued from the burning cathedral by Fr. Jean-Marc Fournier, chaplain of the Paris Fire Brigade. It is now preserved at the Louvre while Notre-Dame is restored.