Answer: Marble

Let’s say you’re tasked with creating a beautiful library that will house rare books and manuscripts, that you want it filled with warm natural light without a single ray of ultraviolet light striking a single page of any of the hundreds of thousands of rare books held within. If not for the natural light requirement, you could just build a bunker-like structure where nary a spine-fading or page-damaging ray of ultraviolet light could land. The lighting requirement, however, complicates the matter. That’s the situation Gordon Bunshaft of Skidmore, Owings & Merrill found himself in when tasked with creating the Beinecke Rare Book & Manuscript Library for Yale University in the 1960s.

The solution? The “windows” of the library are massive slabs of marble from Danby, Vermont, warm in color and rich with veining, quarried to a thinness that renders them translucent. The result is not only practical (indirect light is admitted to the library with no risk of direct sunlight damaging the books), but beautiful: the library is bathed in amber light.