Sainte-Chapelle was built by Louis IX as a reliquary for his collection of apporoximately thirty holy relics which included (he believed) the crown of thorns worn by Jesus at the crucifixion. King Louis IX is reputed to have paid the Venetian pawnbrokers to whom Baldwin II, the Emperor of Constantinople, had pawned the relic, 135 000 livres, to gain possession of this highly prized item. The relic was then stored in a special silver chest which Louis IX commissioned for a further 100 000 livres. To put these figures in perspective, the 13th century cost of building Sainte-Chapelle was only 40 000 livres!
The photograph shows the arch above the entrance to the area in the chapel where the silver chest was kept. It is no longer displayed in the chapel.