It's a name that sounds a wee bit like a Marvel superhero…but it's actually a religious symbol. "Crux" means "cross" (bet more than a few of your knew that already) and "gemmata" means…"gems."
Early on, when Langdon's introduced to Captain Bezu Fache, the professor notices that he's wearing a crux gemmata: In the reflection of the shiny elevator door, he saw the captain's tie clip— a silver crucifix with thirteen embedded pieces of black onyx. Langdon found it vaguely surprising. The symbol was known as a crux gemmata— a cross bearing thirteen gems— a Christian ideogram for Christ and His twelve apostles. Somehow Langdon had not expected the captain of the French police to broadcast his religion so openly. Then again, this was France; Christianity was not a religion here so much as a birthright. (4.52)
This might seem insignificant at the time, but in reality it's important to know that Fache's character's a devout Catholic. This not only helps to set him up as an antagonist for Langdon and Sophie (remember, before the truth is revealed we think the Church is behind everything they're up against, and Fache is obviously gunning to arrest Langdon for crimes he didn't commit) but it also aids in our understanding about why he's so kind to Bishop Aringarosa at the end of the book.