The shell of the pearl oyster Pinctada margaritifera is made up of two layers: an outer layer of calcite prisms and an inner layer of aragonite tablets. Recent studies have shown that the calcite layer develops in a series of steps. We found that the end of prism growth and the start of aragonite deposition are also complex processes. Contrary to the common belief that prism growth is interrupted by the expansion of the aragonite layer, we found that a specific membrane covers the calcite surface before aragonite deposition starts. The earliest aragonite depositions occur as granular spots located only on the surfaces covered by this organic membrane This membrane appears to be the final stage of the calcite biomineralization cycle. This new understanding of calcite development has implications for shell biomineralization research and the production of pearls. [ABSTRACT FROM AUTHOR]

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