japan is where pearls were first cultured in the early 20th century by placing a small bead within the akoya oyster, then submerging the oyster for more than a year in the cool waters of ago bay, about a two-hour drive from kobe. these days there are only a dozen or so small pearl farms at once what was the world epicenter of pearl cultivation.

the process of culturing pearls is a practice more complicated than that of napa valley winemaking. it’s a nuanced science, influenced by intuitive sense and experience. the lower the oyster is placed in the water, the colder the water. this is good, since colder temperatures can trigger a richer lustre in pearls. too low, though, and the amount of oxygen the oyster gets is reduced and that increases mortality. if the oyster grows too fast, the pearl’s shape will be irregular.

in addition to the notorious red tide, an aggressive infestation of plankton, and pollution, the reason for the precipitous drop in japanese pearl production is because of global warming, which has raised the temperature of the waters in ago bay by as much as three degrees over the last decade.

ed note: stephen g. bloom is the author of tears of mermaids: the secret story of pearls (st. martin’s press)