Arcata, CaliforniaOn a drizzly December morning, Damien and Michaela Beauchemin trudge along a redwood trail with their Portuguese water dog, Maui, scanning the duff and listening intently for the sound of a creek. Maui whines and attempts to race ahead, but Michaela holds the dog back. If they move too quickly, they might miss something.

The Beauchemins glance at clues they’ve saved on their smartphones: a riddle about rubbing a belly for luck, a warning about heights, a photograph of ferns. Another saved photo depicts the coveted prize: a large orange-and-white marble nestled amid pebbles in water.

The ornate, glistening sphere is a score in the world of marble hunting, and the Beauchemins—who search for these treasures nearly every weekend—want very much to