By Raymond Zhong

From the bluffs, you might have mistaken the little brown heads poking out of the ocean this month for seals, hundreds of them, gathered for a morning conclave. In fact, they were something almost as surprising: bulb upon glistening bulb of kelp, more of it than this cove in Northern California had seen in years.

Beneath the milky water, the kelp’s ropelike stems stood in thickets dense enough to snag the valve on a scuba tank. The parade of long, luxuriant blades, streaming out of each bulb like wavy tresses, made diving these waters feel a bit like touring a hairdressers’ convention.

One promising product is countertops. A start-up, Primitives Biodesign, is developing a kind of urchin-shell marble, one that Virj Kan, the company’s chief executive, hopes will be strong but lightweight compared with natural stone. A single countertop could use up the urchins that would cover a tennis-court-size area of seafloor, Ms. Kan said. Primitives is aiming to deliver its first units next summer.