How are Akoya pearls formed after nucleus insertion?
This article explains the period between nucleus insertion and harvest, including the difference between one-year and pass-the-year cultivation.
The nucleus is a small, round bead traditionally made from the shell of freshwater mussels found in the Mississippi River basin in the United States. As these shells have become more difficult to obtain, material from freshwater mussels in other regions, including China, may also be used.
The figure shows the typical relationship between the timing of nucleus insertion and harvest.
When nuclei are inserted into Akoya oysters between April and June and the resulting pearls are harvested in January of the following year, those pearls are called “one-year pearls.”
When nuclei are inserted into Akoya oysters between July and December and the resulting pearls are harvested in December of the following year, those pearls are called “pass-the-year pearls.” Inside the oysters, layers of nacre continue to form around the nuclei for more than one year before harvest.
The exact schedule may vary slightly depending on the pearl farm, location, water temperature, and the condition of the oysters.
Because one-year pearls have a shorter cultivation period, their nacre tends to be thinner. However, they are also more likely to retain a round shape.
Pass-the-year pearls have more time to form nacre, so their nacre tends to be thicker. At the same time, the longer cultivation period increases the possibility that they will develop baroque or irregular shapes.
These are general tendencies rather than fixed rules. Pearl quality is also influenced by the oyster’s health, seawater conditions, and the structure of the nacre itself.
Both one-year pearls and pass-the-year pearls have their own strengths and trade-offs.
Pearl processors and retailers often prefer round pearls because they are easier to match for use in strands and generally easier to sell. From that perspective, one-year pearls can be attractive: their shorter cultivation period reduces production costs, and they tend to retain a rounder shape.
Many pearl farmers, however, have traditionally taken pride in producing pass-the-year pearls with thick nacre. Leaving oysters in the sea for another season gives their pearls more time to develop nacre, but it also increases labor and management costs, raises the risk of oyster mortality, and delays the farm’s income.
In recent years, some pearl farms have increased their production of one-year pearls in response to rising costs, cultivation risks, and market demand for round pearls.
This does not mean that one-year pearls are necessarily low quality, or that pass-the-year pearls are always superior. The decision involves a balance between nacre thickness, shape, production risk, cost, and what the market is prepared to buy.
For example, a pearl farm can increase the proportion of one-year pearls by extending the early harvest to include some pearls from oysters that received nuclei in July or August. These pearls are harvested in January rather than being allowed to continue growing inside the oysters until the following winter.
However, increasing the proportion of one-year pearls automatically reduces the number of oysters available to produce pass-the-year pearls in the next season.
Once a farm has shifted toward one-year production, increasing pass-the-year production again is not easy. The farm must leave more oysters in the sea and postpone the income from those pearls for another season.
In that sense, increasing one-year production is similar to borrowing against a future harvest and its income.
There is a practical limit to the number of oysters that can receive nuclei each year. A farm cannot immediately replace the pearls it has already harvested simply by inserting more nuclei.
For this reason, deciding whether to harvest pearls during the coming winter or leave the oysters in the sea for another year is not only a question of pearl quality. It is also a major financial decision for the pearl farm—a choice between receiving income now and investing in the possibility of thicker nacre in the future.
Explore the History of Akoya Pearls
Akoya pearls have a long history in Japan, shaped by scientific discovery, the development of cultured pearl farming, and generations of people who worked with the sea.
If you would like to discover how Akoya pearls came to be known—and how they eventually changed the world—you may enjoy this article.
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