Question

The following passage is adapted from an essay on medieval art and architecture:

To the medieval builder, light was not merely a physical phenomenon that illuminated stone; it was a primary theological medium. When Abbot Suger oversaw the rebuilding of the Abbey Church of Saint-Denis in the twelfth century, he sought to create an architecture of transport. He believed that by flooding the nave with *lux nova*—new light—worshipers would be elevated from the material world to the immaterial realm of the divine. This was achieved by replacing the heavy, load-bearing walls of the Romanesque style with a skeletal framework of pointed arches and ribbed vaults. This skeletal system, reinforced by external flying buttresses, allowed builders to open up vast expanses of wall space for stained-glass windows.

As a result, the interior of the cathedral became a kaleidoscope of shifting hues. The light did not shine directly, but was filtered through intense blues, deep reds, and brilliant golds, casting an ethereal glow over the stone surfaces. The writer Georges Duby described this effect as a 'curtain of gems' suspended between the earthly congregation and the heavens. This curtain did not block the view of the outside world so much as it transformed the nature of sight itself, turning physical vision into a form of spiritual contemplation. By standing within this enclosure of colored light, the medieval viewer was meant to feel suspended in a transitional space, caught between earth and heaven.

As it is used in the second paragraph, the metaphorical phrase 'curtain of gems' refers to which of the following?