Jean des Esseintes is the last member of a powerful and once proud noble family. He has lived an extremely decadent life in Paris, which has left him disgusted with human society. Without telling anyone, he retreats to a house in the countryside.
He fills the house with his eclectic art collection (which notably consists of reprints of paintings of Gustave Moreau). Drawing from the theme of Gustave Flaubert's Bouvard et Pécuchet, Des Esseintes decides to spend the rest of his life in intellectual and aesthetic contemplation. Throughout his intellectual experiments, he recalls various debauched events and love affairs of his past in Paris.
He conducts a survey of French and Latin literature, rejecting the works approved by the mainstream critics of his day. Among French authors, he shows nothing but contempt for the Romantics but adores the poetry of Baudelaire and that of the nascent Symbolist movement of Paul Verlaine, Tristan Corbière and Stéphane Mallarmé, as well as the decadent fiction of the unorthodox Catholic writers Auguste Villiers de l'Isle-Adam and Barbey d'Aurevilly. He rejects the academically respectable Latin authors of the "Golden Age" such as Virgil and Cicero, preferring later "Silver Age" writers such as Petronius and Apuleius as well as works of early Christian literature, whose style was usually dismissed as the "barbarous" product of the Dark Ages. Schopenhauer, he exclaims, has seen the truth, and he clearly expressed it in his philosophy. He studies Moreau's paintings, he tries his hand at inventing perfumes, and he creates a garden of poisonous flowers. In one of the book's most surrealistic episodes, he has gemstones set in the shell of a tortoise. The extra weight on the creature's back causes its death. In another episode, he decides to visit London after reading the novels of Dickens. He dines at an English restaurant in Paris while waiting for his train and is delighted by the resemblance of the people to his notions derived from literature. He then cancels his trip and returns home, convinced that only disillusion would await him if he were to follow through with his plans.
Eventually, his late nights and idiosyncratic diet take their toll on his health, requiring him to return to Paris or to forfeit his life. In the last lines of the book, he compares his return to human society to that of a non-believer trying to embrace religion.

Question: Where does he put gemstones?
Answer: On the shell of a tortoise.
Alternative answer: A tortoise's shell.