In Chapter 2, Conrad writes:
"And the lofty frontal bone of Mr. Kurtz! They say the hair goes on growing sometimes, but this—ah—specimen, was impressively bald. The wilderness had patted him on the head, and, behold, it was like a ball—an ivory ball; it had caressed him, and—lo!—he had withered; it had taken him, loved him, embraced him, got into his veins...."
Kurtz has devoted his life to procuring ivory with the result that he has come to resemble ivory himself.