Keshi pearls are formed when the oyster rejects and spits out the implanted nucleus before the culturing process is complete, or the implanted mantle tissue fractures and forms separate pearl sacs without nuclei. These pearl sacs eventually produce pearls without a nucleus.

Keshi may form in either saltwater or freshwater pearls. They are generally small in size and, because there was no nucleus to guide the ultimate shaping of the pearl, their shapes vary widely. Keshi come in a wide variety of colors, and tend to have high luster and even rare orient. This is due to their solid-nacre composition.

Because the implanted nucleus of the pearl has been expelled by the oyster, the resulting keshi pearl is 100% nacre. This gives it an especially lustrous and shimmering surface quality. Most keshi, in fact, have a greater luster than even the best-quality cultured pearls. The fact that keshi pearl are solid nacre does not, however, give them the classification of natural pearls. This is because keshi are a bi-product of the culturing process, not a natural occurrence.

Keshi pearls, especially Tahitian and South Sea, were once quite the bargain yet beautiful and unique pieces. Today Keshi pearls are much more rare. This is because Tahitian and South Sea pearl farms are now x-raying oysters to determine whether or not the nucleus has been expelled. When a nucleus-free oyster is found they are then re nucleated before a keshi has time to form. This practice has made keshi pearls much more of a rare find than they once used to be.

The word keshi means "poppy seed" in Japanese, and these pearls are often also referred to as "poppy seed pearls.