The book uses an oxymoron to describe Doña Maria's difficult and lonely upbringing:
"...her mother persecuted her with sarcasms in an effort to arouse some social charms and forced her to go about the town in a veritable harness of jewels."
This passage is describing Doña Maria's mother's harsh attitude and the clothes she was forced to wear in public. The phrase "harness of jewels" may not seem like an oxymoron, since the words do not have inherently opposite meanings. But in the context of this passage, Wilder draws on the words' implicit connotations to create an oxymoron. A harness, a device worn by horses and other livestock animals, signifies bondage or servitude. Meanwhile, jewels represent Doña Maria and her family's wealth and class status. By describing Doña Maria's jewels, the symbol of her privilege, as a harness, the novel suggests that belonging to this class is a form of servitude for Doña Maria: because of her looks and personality, she is unable to fit in, has difficulty finding a husband, and is mocked by her peers. For much of the novel, Doña Maria lives in a state of emotional bondage to her daughter, who consistently rejects her love because she resents her mother's eccentric behavior and refusal to defer to social norms. The oxymoron in this paragraph helps to establish that narrative arc.
It's also important that Wilder is using modern, somewhat skeptical syntax to describe a style of dress that was already antiquated at the time of writing. By describing Doña Maria's attire as a "harness of jewels," he invites the reader to chuckle at modes of dress that now seem strange or even funny. In that sense, this oxymoron is a stylistic reminder that the novel is not attempting to inhabit an 18th-century sensibility or consciousness, but consciously using the historical setting to explore questions of divine will and human agency that are relevant in any time period.
In one of Doña Maria's letters to Doña Clara, she writes that she has enclosed a special gold chain as a present for one of her daughter's friends. Doña Maria tells a fantastic (and obviously false) story about how she obtained the necklace, writing that she walked into a painting by Diego Velázquez, a famous Spanish artist, and took the necklace from one of its subjects:
The canvas resisted for a moment, but the painter himself came forward to lift me through the pigment. I told him that the most beautiful girl in Spain wished to present the finest gold chain that could be found to the most gracious king in the world. It was as simple as that...
The image of Doña Maria trying to enter the resistant canvas, and of the painter bringing her "through the pigment," appeal to the reader's imagination because they conjure up a fantastic world that obviously does not exist. The romantic concept of being able to literally penetrate a work of art and interact the subjects it represents elevates the gold chain from a simple object to a powerful talisman.
It's important to remember that in the context of the novel, Doña Maria has imagined this scenario and written these words herself. The beautiful image she casually creates in this letter demonstrates her power as an artist, which is important to the broader arc of the novel: Wilder repeatedly tells the reader that Doña Maria's correspondence will become a celebrated piece of national literature after her death.
At the same time, the image demonstrate Doña Maria's desperation to earn her daughter's love and the narcissism that fuels her constant letter-writing. To elicit Doña Clara's approval, Doña Maria inflates her own actions far beyond what she actually did, describing herself interacting with a great work of art rather than simply buying a gold chain. As Wilder says elsewhere in the novel, Doña Maria's own ego keeps her trapped in this ingratiating mode of address and limits what she can achieve in her art. Only when Doña Maria learns to love her daughter selflessly is she able to achieve peace and tranquility. In her last letter to Doña Clara, written shortly before her death in the bridge collapse, Doña Maria finally transcends with and style to create a truly great work of art.
The book's first description of the Archbishop uses visual imagery to help the reader envision the priest's unusual experience and gain an initial understanding of his character:
There was something in Lima that was wrapped up in yards of violet satin from which protruded a great dropsical head and two fat pearly hands; and that was its archbishop. Between the rolls of flesh that surrounded them looked out two black eyes speaking discomfort, kindliness and wit.
The description starts by referring to the Archbishop as an object ("something") rather than a person, a style choice that encourages the reader to view the priest not as an ordinary man but a strange, almost fantastical creature. He is composed of constituent parts: "fat pearly hands," black eyes, and a "dropsical" or swollen head. Likewise, instead of saying explicitly that the Archbishop wears the purple robes typical of a high-ranking priest, the novel describes his attire as "yards of violent satin." This language defamiliarizes the priest's vestments in order to make them more visually striking to the reader. Like the book's previous description of Doña Maria's "harness of jewels," the imagery in this passage presents antiquated modes of dress to a modern reader in a manner that is easy to visualize yet also feels fresh and new.
However, it's important to note that this passage also demonstrates the limits of Wilder's sympathetic humanism. The novel derives humor from the archbishop's fatness and uses his weight as a shorthand for his many vices, such as greed and extravagance. While such flaws have often historically been associated with fat people, the contemporary reader can see that this method of characterization provides little insight into the archbishop's psyche and stigmatizes a certain body type.