Sainte Chapelle was originally built to house relics of the Passion of the Christ acquired by the monarch during the 13th century. These include a piece of the True Cross and the Holy Crown, which nearly disappeared in the early 19th century.
Starting in 1790, Sainte Chapelle was restricted to members of the clergy and was emptied of its contents. It progressively became archive storage for the neighboring Palais de Justice - Paris’s Court House - until a restoration campaign was started under pressure from the public.
Today, it is no longer possible to admire all the original relics for which Sainte-Chapelle was built, as they were either lost or displaced during the French Revolution. However, visiting the chapel is an experience in itself!