Monday was market day for Stella McCartney, who turned her show at Paris Fashion Week into a street fair celebrating a few of her favorite things.

The designer took over the Marché Saxe-Breteuil, not far from the Eiffel Tower, and transformed it into Stella’s Sustainable Market for the day. Like regular market hawkers, the minders of the 21 stalls beckoned onlookers with their wares.

At the Keel Labs stand, cofounder and chief executive officer Tessa Callaghan was preparing to unveil the fruit of six years’ labor. The start-up’s seaweed-based yarn was making its global debut on the runway in the form of a crochet knit dress and tunic dotted with mirrored discs that sent rays of light bouncing off the sidewalk.

“This is the biggest deal,” Callaghan said. “I guarantee I’ll be crying later.”

It summarized the spirit of the collection, which McCartney wanted to be not just eco-friendly but fun.

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