Sainte Chapelle (the “Holy Chapel”) was built in the 13th century to house Louis IX’s relics, including the Crown of Thorns and a fragment of the True Cross. Sainte Chapelle was built in six years, which just blows my mind. I mean, there are photo albums and scrapbook projects I’ve had around for much, much longer than that.
During the French Revolution, the relics were kept in the National Library, and in 1806, were given to the archbishop of Paris to be placed in the Cathedral treasury. The Crown of Thorns is now housed at Notre Dame, and is put on display the first Friday of each month and every Friday during Lent.