Sainte-Chapelle, 1241-1248, Île de la Cité, Paris, Photo by Didier B, Creative Commons Attribution-ShareAlike 2.5 Generic via Wikipedia.
Built by French King Louis IX, a.k.a. St. Louis, in the mid-13th century, Sainte-Chapelle almost itself is a reliquary rather than a chapel to house reliquaries. The space is connected to the Royal Palace so that the royal family could simply walk into the second floor of the chapel from their living space.
Louis IX built the chapel to house prized relics from the days leading up to the death of Jesus which is referred to as Christ’s Passion. From his cousin Baldwin II, who was the emperor of Constantinople, Louis acquired the Crown of Thorns, a piece of the True Cross, the iron lance, the sponge, and a nail.
This royal Gothic chapel defined the Rayonnant style of architecture with its intricately decorated walls and vaults. The thin, gold-covered ribs open the walls to allow for extensive use of stained glass that fills the space with colored light. The jewel tones in the windows extend to the celestial blue colors on the ceiling, transforming the space into a type of heaven on earth.
