The Most Influential Piece of Amber since Jurassic Park
This small necklace of amber beads belonged to Mortenson's sister, Christa. She develops a severe case of meningitis, which eventually takes her life, and Mortenson wants to put the necklace at the top of K2 in tribute to her. We're not sure why, because she wasn't a mountain climber—maybe 28,267 (the height of the mountain, in feet) was Christa's favorite number or something.
It's a good thing Christa had this favorite necklace instead of, say, a favorite anvil or steel safe, because there's no way in heck Mortenson would have attempted to get either of those things to the top of a mountain. And were it not for the necklace, Mortenson would never stumble into Korphe on that fateful day he makes his way back down the mountain, prompting the mission that changes his life—and the lives of others—forever.
So while he doesn't leave Christa's necklace at the summit, there are now dozens of schools that exist because of her. But since he didn't leave it at the top of the mountain, what did he do with it? No clue, Shmoopers; no clue.