There are several different facets (no pun intended) to the symbolism of the infamous glass pyramid outside Paris's Louvre museum.
On the one hand, it's a symbol in and of itself:
The new entrance to the Paris Louvre had become almost as famous as the museum itself. The controversial, neomodern glass pyramid designed by Chinese-born American architect I. M. Pei still evoked scorn from traditionalists who felt it destroyed the dignity of the Renaissance courtyard. Goethe had described architecture as frozen music, and Pei's critics described this pyramid as fingernails on a chalkboard. Progressive admirers, though, hailed Pei's seventy-one-foot-tall transparent pyramid as a dazzling synergy of ancient structure and modern method— a symbolic link between the old and new— helping usher the Louvre into the next millennium. (3.32)
Langdon can't see anything without thinking about the symbolic imagery behind it (a career hazard, we suppose), so it's a little surprising that it took him so long to come to the revelation he has at the end when he pieces together Saunière's final clue to the Grail's resting place.
When he's talking to Marie Chauvel, she even draws it out for him:
"The blade and chalice?" Marie asked. […] "Ah, yes, of course. The blade represents all that is masculine. I believe it is drawn like this, no?" Using her index finger, she traced a shape on her palm.
"Yes," Langdon said. Marie had drawn the less common "closed" form of the blade, although Langdon had seen the symbol portrayed both ways.
"And the inverse," she said, drawing again on her palm, "is the chalice, which represents the feminine."
"Correct," Langdon said. (105.34-41)
So, when he finally realizes that Saunière's hidden the Grail beneath the inverted pyramid and its matching counterpart that is hiding in plain sight in the Louvre, ("the blade and chalice guarding o'er her gate") we all kind of want to smack our foreheads and yell, "D'Oh!"