Chinese freshwater pearls with lost nuclei
A curious variety of freshwater pearls, rather flat and lumpy in a distinctive way, as are pearls nucleated with irregular shaped pearls, are surprisingly lightweight for their size. This turns out to be because they are hollow. The explanation is they are nucleated with a soluble material, usually termed "mud", most of which may be removed after holes are drilled in the pearls. Some variation on this theme accounts for pearls that are somewhat hollow without a drill hole. This implies the use as a nucleus of some substance that is absorbed or leaches through the pearl's surface. Purists (including most ocean pearl producers) refuse to recognize hollow pearls of this type as pearls, because the fail the test of depending on only shelled creatures and shell nuclei for production. They reject lightweight coin pearls by the same token. But many wish to exclude every type of Chinese pearl, and this is unrealistic. Little is published about how hollow pearls, dubbed "souffle pearls" by enthusiast Jack Lynch, wear compared to other freshwater pearl varieties. Many writers seem captivated by their color range, which must exceed that of any pearl, one speculating that "metal oxides in the mud" might be responsible for some of the more unnatural hues that they have observed.
When I first heard of "souffle pearls" I thought of pearls which are inflated due to the presence of gassy microbes on their surface, an unwanted phenomenon because it renders pearls unstable. I have inadvertently crushed such pearls in the pearl holding vise of the drill press. Though the ways in which hollow pearls become hollow may be less like cooking a souffle, the naming is consistent with the use of food names (rice, corn, potato) to describe pearl shapes and will probably prevail. Meanwhile, I look forward to more informative publications about hollow pearls from people more experience with them than I.