One day he was working with a road gang clearing rocks from a lava field. Sitting alone during his lunch break, he was surprised when a strange man dressed in gray and covered with wiry hair popped up from behind a boulder.

“Are you Jón Jónsson?” asked the stranger.

“I am,” Jón said. “And who might you be?”

“My name is Tyr, and I have a gift for you.”

“And why would you give me a gift?” asked Jón. “I’ve never met you before.”

“Never mind that,” Tyr said. “Here it is.” From behind his back he produced a small, black box made of obsidian. “It’s yours if you want it. I have only one condition: if you should earn any money by use of it, give me ten percent.”

It was finely crafted and quite beautiful, and Jón thought perhaps he could sell it. As to why Tyr couldn’t simply sell the box himself, Jón guessed he was a criminal of some sort, and to show his face in a town would’ve been too dangerous.

“Five percent and you’ve got a deal,” said Jón—not because ten percent was too much, but because he liked to feel he’d gotten the upper hand in every transaction.

“All right,” said Tyr, and he handed over the box so quickly that Jón wished he’d demanded two and a half percent instead of five. Before he could renegotiate, Tyr had ducked behind the boulder again, and when Jón went to look for him, he saw only a puff of smoke lingering in the air.

Jón tucked the black box into his knapsack, and at the end of the workday, he went into the town of Egilsstaðir to try and sell it. He offered it first to Grímur Snorrisson, the jeweler, but Grímur didn’t know what to make of it. “Obsidian’s wonderful for making knives,” the jeweler said, “but why would anyone carve a box from it?”

“I’ve no idea,” said Jón, “but I’ll sell it to you for ten crowns.”

“You must be out of your mind!” said Grímur. “It isn’t worth two.”

Next he tried to sell it to Steffi Ólafsdóttir, the wealthiest woman in town, for eight crowns, but she told Jón to go jump in a volcano. In desperation he cut his price to five crowns and offered it to Sveinn Swansson, who dealt in rare and precious objects, but though Sveinn said he’d never seen anything like it, he claimed to be short on cash. “Would you take four?” Sveinn offered.

“I won’t be swindled!” Jón declared, and he stalked off with the box wedged tightly under his arm.

The sun was setting as he trudged back to the simple boarding-house where he was staying. Because the stingy landlord had only given Jón a single candlestick to light his room at night, Jón used his old trick and swiped a bit of fading daylight. (He took it from behind a sheep pen where no one ever went; he didn’t want anyone to see a strange patch of black hanging in the air and start asking questions.) He snuck up to his room with the light clenched tightly in his fist. He’d meant to wait until it was good and dark to release it, but after a few minutes there was a knock at his door. It was the landlord, wanting to know if Jón could come outside to help him round up a cow that had wandered into the wrong field.