A cultured pearl contains a nucleus.
A Keshi pearl does not contain a nucleus.
My all-time favourite pearl is the keshi, a pearl solid in nacre not containing a nucleus. Technically they are produced within the cultured pearl farming industry and considered a by-product. Keshi are bright in lustre and irregular in shape, a pearl organic in appearance.
These very individual gems are priced by weight as opposed to size and the smoother and bigger they are the higher per gram price.
The term keshi means ‘poppy seed’ in Japanese, a reference to the tiny size found in the Japanese Akoya oyster, which are seldom more than a few millimeters in diameter.
Rare South Sea keshi pearls can be large and may require an xray to determine if it does indeed contain a nucleus, particularly if it is roundish and smooth because it will be extremely rare and valuable but near impossible to distinguish from a cultured pearl.
Freshwater keshi pearls are not as rare, being a deliberate non-nucleated cultivation inserting mantle tissue into soft side tissue of an oyster but don’t let that put you off. High grade AAAA freshwater keshi pearls have the same tactile shiny presence for a fraction of the price.