Keshi pearls are quite unique in the way they are formed, when compared to the standard method of pearl formation.
Usually, a pearl is formed when a substance such as a piece of shell becomes embedded in a mollusk such as an oyster. A type of infection ensues, and the oyster heals itself by developing a layer of nacre over the intruding substance, walling it from irritating further. Layers of nacre continue to form as long as the oyster is alive, and the result is a pearl.
However, with Keshi pearls, there are two ways that the pearl can form, none of which has to do with the “usual” process as described above. The first way that the keshi pearl can form occurs when the oyster rejects and spits out the substance – the nucleus – before the development of the pearl has had the opportunity to finish. The second possibility is that the nucleus fractures and forms two separate pearl sacs – that is, circles of nacre – neither with a nucleus. Eventually, one or two nucleus-less pearls will usually form.
Keshi pearls – also known as poppy seed pearls or seed pearls, as keshi means poppy seed in Japanese – may result from either freshwater or saltwater pearls. Keshi pearls are usually quite small in size, and as they do not have nuclei – upon which the shape of a pearl is usually determined – there is usually a large degree of variance in the shape of keshi pearls. Keshi pearls also come in a variety of different colors and shades, and are known for their luster and uncommon orient. This is a result of their composition consisting of solid nacre. As the nucleus is eventually expelled by the oyster before the pearl is actually formed, the keshi pearl consists entirely of nacre.
Though this is true, keshi pearls are not given the classification of being a natural pearl. The reason is that keshi pearls are actually a bi-product of the culturing process, and not something that happens naturally.
At one time, keshi pearls, especially of Tahiti and the South Sea, could be procured at quite the bargain; even for the most beautiful and rare samples. However, today keshi pearls are considered to be much more exceptional and infrequent. The reason for this is that both Tahitian and South Sea pearl farmers now x-ray their oysters to see whether or not the nucleus is still inside, or if it has been expelled. Should an oyster be found which has expelled its nucleus, it is re-nucleated before a keshi pearl would have the opportunity to develop. Thus the keshi pearls have become even more rare than they once were.
Still, keshi pearls remain popular as they are made of solid nacre and therefore usually have quite a bright luster. Their variable shapes are frequently considered quite desirable to jewelers who wish to design something unique and innovative.