Calandrino’s search for the Heliotrope turns a fantasy of sudden wealth into a cruel domestic farce: tricked into believing that ordinary black stones make him invisible, he carries a houseful home and beats Mistress Tessa when she sees him. Told by Elisa as the third story of the Eighth Day, the episode makes the prank’s comic machinery inseparable from its consequences. Calandrino’s confidence in the magic is so complete that even the pain inflicted by his companions becomes, to him, proof that the stone has worked.

The promised stone and the plan

In Florence, Calandrino is a painter known for his simple wits and unusual ways. He spends much of his time with Bruno and Buffalmacco, fellow painters whose good sense contrasts with his credulity and who enjoy his eccentricities. Another Florentine, the witty and resourceful Maso del Saggio, hears of Calandrino’s simplicity and decides to amuse himself at his expense. In the church of San Giovanni, he describes marvels with an assured air, including a stone called Heliotrope. Its supposed virtue is that whoever carries it cannot be seen while it is on his person. Maso says it can be found in the Mugnone and describes it as nearly black, though of varying sizes.

The promise catches Calandrino’s imagination at once. He leaves Maso intending to look for the stone, but will not undertake the search without Bruno and Buffalmacco, whom he particularly likes. After spending the morning looking for them, he finds them working at the Ladies’ Convent at Faenza. He presents the discovery as a route to becoming the richest men in Florence: with invisibility, they could take coins from the moneychangers’ tables without being noticed. His plan reveals both his confidence in the tale and his readiness to turn its supposed magic to theft.

Bruno and Buffalmacco recognize the deception and conceal their amusement. When Calandrino cannot remember the stone’s name, he insists that its name does not matter, since they know its power. He proposes collecting every black stone they find. Bruno delays the expedition, claiming that the high sun has dried the stones so that some appear white, and that people along the river might discover what the three are doing. He recommends a Sunday morning, when the color will be easier to judge and fewer people will be present. Calandrino accepts the explanation and insists that the supposed secret remain confidential.

The search along the Mugnone

At daybreak on Sunday, Calandrino and his two companions leave Florence by the San Gallo Gate and descend into the bed of the Mugnone. Calandrino, the most eager, ranges ahead, picking up every black stone he sees and stuffing it into his clothing. When his bosom is full, he gathers up his gown to make a pouch, then fills his mantle as well. Bruno and Buffalmacco also collect stones, but their real purpose is to make Calandrino believe that he has disappeared from their sight.

When they have gone some distance, Bruno asks where Calandrino is. Buffalmacco pretends not to know, though he sees him nearby. The two then act as if Calandrino has gone home and left them foolishly searching. Within hearing, Calandrino takes their performance as confirmation: because they cannot see him, he believes he has found the Heliotrope. He turns back without answering them, delighted by what he takes to be good fortune.

The joke becomes physical as the companions follow him up the riverbed. Bruno throws a stone at Calandrino’s shins, and Buffalmacco strikes him in the back with another. Calandrino feels both blows, lifts his leg and puffs from the pain, but keeps silent. The blows do not dislodge his belief; instead, he interprets his companions’ apparent failure to see him as proof that he is invisible. At the San Gallo Gate, Bruno and Buffalmacco discard their own stones and pause at the customs house, while Calandrino continues toward home under the weight of his collection.

The supposed magic fails at home

The gate officers, forewarned by Bruno and Buffalmacco, pretend not to see Calandrino and let him pass, laughing at the trick. Fortune further assists the deception: few people are abroad because it is nearly dinner time, and none stops him on the way to his house near Canto alla Macina. Laden with rocks, he reaches the stairhead, where Mistress Tessa is waiting. She complains that he has returned late, after everyone else has already eaten. Her ordinary greeting punctures his certainty: he has been seen.

Calandrino immediately decides that Mistress Tessa has undone the stone’s virtue. He rushes upstairs and unloads the stones into a small room, then grabs his wife by the hair, throws her down and beats and kicks her. She begs for mercy with clasped hands, but he continues until he is exhausted. His anger falls not on the men who engineered the deception, but on the person whose presence, in his mistaken account, has exposed him.

Bruno and Buffalmacco arrive after hearing the beating. They find the room full of stones and Mistress Tessa bruised, dishevelled and weeping in a corner; Calandrino sits apart, panting and red with anger. He cannot at first explain himself in more than broken words. Buffalmacco instead accuses him of leaving the others behind in the Mugnone, sustaining the performance that Calandrino has tricked them by going home unseen.

The prank’s aftermath

Calandrino then recounts the search from beginning to end. He explains that he was close by when the others asked where he had gone, that he went ahead of them, and that the stones they threw struck his shins and back. He also cites his passage through the gate and the silence of acquaintances who usually stop him to invite him for a drink as evidence that nobody could see him. In his version, the officers and friends all confirm the stone’s power by failing to acknowledge him; Mistress Tessa alone has broken it by appearing before him.

As his anger rises again, Calandrino threatens to beat her once more. Bruno and Buffalmacco restrain him and say that he alone is responsible: if women can make such things lose their virtue, he should have warned his wife not to appear. Their explanation continues the deception even as they stop its immediate violence. They eventually make peace between the couple, though only with difficulty, and leave Calandrino disconsolate in a house still filled with stones.

The ending holds the trick’s two consequences together. Calandrino has gained no invisibility or wealth; he has only collected burdensome rocks and been struck by his own companions. Yet his conviction survives long enough to redirect his frustration into violence against Mistress Tessa. The episode’s comic reversal depends on the gap between what the reader sees—the companions’ staged deception—and what Calandrino believes: that a magical stone has worked until a woman destroys its power. Elisa’s tale makes that gap the source of the joke, while the battered wife and the final roomful of stones disclose its cost.