Joseph Conrad (1857–1924), author of the Heart of Darkness, personally spent four months sailing on the Congo River in 1890. At that time, colonists were active in Congo and elsewhere in Africa. Conrad's experience had a profound impact on him and later inspired him to write his most famous story. In 1979, it served as the inspiration for the film Apocalypse Now.
In Heart of Darkness, the protagonist is Charles Marlow. He tells the story of his journey on Congo's rivers, during which he witnesses the brutal exploitation of Africans by white colonists. As he travels, he hears about a godlike Mr. Kurtz, the manager of an ivory trading station. Kurtz is known to the outside world as a "civilizing" influence in Africa. In fact, the mysterious and tyrannical Kurtz is the personification of the greed, brutality, and exploitation of Western imperialism.
Ivory was sought by outsiders arriving in Congo in the late nineteenth century. Avaricious and ruthless men went there because elephants were still plentiful. Not only did they slaughter elephants, they also had a pernicious effect on the native societies on the region. They even kidnapped natives for nefarious purposes. For these reasons, ivory symbolizes imperialism in the Heart of Darkness.
In summary, Heart of Darkness is a powerful indictment of Western imperialism.