How does ivory appear as an ironic symbol in the novel Heart of Darkness by Joseph Conrad?

In Joseph Conrad's novel, Heart of
Darkness, ivory is an ironic symbol.

Marlow is
the main character and the narrator of the story. He has been hired by the Company to go
deep into the territory that was then known as the Congo in order to retrieve Kurtz, an
agent for the organization that has not been out of the jungle for over a
year.

The Company is becoming richer and richer in this new
"empire," where they ship natural resources out of Africa back to Europe—especially
ivory which is used to make things like fans, piano keys and billiard (pool) balls.
Ivory is owned by the well-to-do, and there is a great demand for
it.

While the civilized and wealthy enjoy the presence of
ivory in their daily lives, the cost of obtaining the ivory is enormous—most especially
in terms of human life. Marlow sees terrible things on his journey. He sees blatant
destruction at the Lower Station, where men are blasting dynamite for no apparent
purpose. Machinery is discarded all over, rusting in the grass. Worst of all, the
natives are enslaved by the Company: they are shackled, starving and sorely
mistreated—they appear to Marlow like walking death. These men are treated like animals,
and the death of a native is barely noticed—all for the sake of the
ivory.

When Marlow finally arrives at the Inner Station,
and catches up with the brilliant, admired and successful Kurtz—who has collected and
shipped more ivory than all of the other agents combined—Marlow is appalled at what he
finds. Outside of Kurtz's living and working quarters are poles that Marlow first thinks
are wooden decorations.

readability="11">

...its first result was to make me throw my head
back as if before a blow. Then I went carefully from post to post with my glass, and I
saw my mistake...food for thought and also for the vultures...They would have been even
more impressive, those heads on the stakes, if their faces had not been turned to the
house.

Marlow discovers that
the natives worship Kurtz like a god. There is evidence of human sacrifice, and Kurtz
himself is mad. He is also unwell, and dies not long after Marlow forcibly removes him
from the station.

Ivory is white, which is a color,
ironically, symbolic of purity. Ivory had many elegant uses and was highly prized by
those wealthy enough to own objects made of it. It is an ironic symbol because as much
as it symbolizes wealth and success, it also symbolizes "moral corruption," madness and
death. While it served to build one empire, it was also destroying the land, culture and
people of the Congo.