Chloe Stein, an executive chef and caterer, remembers fondly her fairy tale engagement in early September 2016.

She and her then-boyfriend, Deepak Panjwani, a data analyst at Bloomberg, were vacationing in Sweden. They took a day trip to Drottningholm Palace, the private residence of the Swedish royal family, and toured the vast 16th-century gardens.

At a scenic overlook, Mr. Panjwani surprised Ms. Stein by proposing marriage. But instead of presenting a traditional diamond solitaire, he held out a “smog free ring,” a piece of designer jewelry that has come to symbolize the fight against urban pollution.

The ring is made of hundreds of thousands of gallons of pollution sucked from the air and compressed into a tiny box and covered by a shiny, protective case. (It’s essentially a black mass inside a clear cube.) The particles in the ring are considered so dangerous that if breathed in, they can shorten an adult’s life expectancy by six to eight years, according to the ring’s designer and creator, Daan Roosegaarde, a Dutch artist and technologist.