In Book 2, the Prices get to know the schoolteacher Anatole better when he comes to their home for dinner. Rachel is perplexed when Anatole says he "spent some time at the diamond mines down south in Katanga," and her confusion is an example of situational irony. With allusions and her Southern dialect, she describes how Anatole has presented an entirely different idea of diamonds than the one she's used to:
When he spoke of diamonds I naturally thought of Marilyn Monroe in her long gloves and pursey lips whispering “Diamonds Are a Girl’s Best Friend.” My best friend Dee Dee Baker and I have snuck off to see M.M. and Brigitte Bardot both at the matinee (Father would flatout kill me if he knew), so you see I know a thing or two about diamonds. But when I looked at Anatole’s wrinkled brown knuckles and pinkish palms, I pictured hands like those digging diamonds out of the Congo dirt and got to thinking, Gee, does Marilyn Monroe even know where they come from? Just picturing her in her satin gown and a Congolese diamond digger in the same universe gave me the weebie jeebies.
Rachel alludes to American star Marilyn Monroe, who had recently appeared in the musical film Gentlemen Prefer Blondes, in which she sings a jazz song titled "Diamonds Are a Girl's Best Friend." This allusion sets up that Rachel's understanding of the world is heavily influenced by her time spent in the United States. She never considered that diamonds had to be mined until she meets Anatole, and when she learns this reality, she experiences cognitive dissonance. How can Marilyn Monroe's diamonds come from the Congo when the Congo is the opposite of glamor and wealth? Her Southern teenage dialect ("Father would flatout kill me," "Gee," "weebie jeebies") also illustrates her foreignness to the Congo, as well as her naïveté. Rachel's time in the Congo is reshaping everything she thought she understood, including diamonds.
In Book 4, village chief Tata Ndu interrupts one of Nathan's sermons to hold an election on Jesus Christ. In The Poisonwood Bible, democracy and Christianity are both ideologies that Westerners encourage Africans to adopt. By combining them, Tata Ndu undermines Nathan's power and generates situational irony: the two programs seem to have a fundamental incompatibility.
Tata Ndu turned directly to Father and spoke to him in surprisingly careful English, rolling his r’s, placing every syllable like a stone in a hand. “Tata Price, white men have brought us many programs to improve our thinking,” he said. “The program of Jesus and the program of elections. You say these things are good. You cannot say now they are not good.”
By setting up an election for Jesus, Tata Ndu points out the conflict between Christianity and democracy, which are both ideologies brought from more developed Western nations to Africa. A simile compares Ndu's careful pronouncement of his English words to the precise placement of stones. This simile suggests Ndu's words have weight, like rocks, and that he handles them exactly and mindfully. He wants his message completely understood.
When Tata Ndu tells Nathan that "You say these things are good," he speaks both to Nathan as an evangelical, who has insisted that civilized people are Christians with democracies, and to foreigners who hope to "civilize" Africa in general. The election on Jesus generates situational irony because, despite insisting both Christianity and democracy are good, Nathan also insists they cannot be combined in this way. He also can't offer a coherent reason as to why they cannot be combined. Tata Ndu undermines Western imperial ideology by taking its arguments to their logical extreme, a process that leads to the democratic village-wide rejection of Christianity. Tata Ndu works within the Western framework he's been presented, but in doing so, he makes that same framework seem arbitrary and half-baked.
It's worth mentioning that Christianity and democracy have both coexisted and clashed over Western history. A more thoughtful or patient evangelist than Nathan might have been able to explain why voting on Jesus is a bad idea. But what Ndu rejects here is not necessarily democracy or even Christianity. Instead, he rejects Nathan's self-righteousness and conviction that Americans are more intelligent, civilized, and logical than the Congolese villagers.