Von Rumpel has discovered that he has a tumor, probably brought on by his exposure to mustard gas during the war. He takes medicines to reduce the growth of the tumor, but the medicines make him very ill. In spite of his sickness, he researches the history of the Sea of Flames. He reads a book explaining that the stone can make its owner live forever.
We see where Doerr is going with this: von Rumpel seems to be dying because of a growing tumor in his body, and so he’s going to hunt down the diamond to try and give himself eternal life. This takes the Sea of Flames plot to a new level, as von Rumpel becomes a more grotesque figure, and the “curse” of the diamond becomes real for all intents and purposes—as long as this man is willing to kill for its “power.”