Pearl nacre is the coating that a pearl oyster produces to cover pearl nucleus. Pearl nacre is composed of thousands of layers of thin calcium carbonate (CaCO3) crystalline substance with the thickness of 0.35 to 0.5 µm (1µm = 1/1000mm). A cultured pearl is formed by inserting a nuclear as an irritant inside a live oyster. The oyster coats the nuclear by depositing repeated layers of nacre formed by secretions from the animal’s mantle tissue. The number and quality of these nacre layers affect the quality of the pearl.
Nacre thickness is the most important factor to consider when choosing cultured pearls. It determines the durability and elasticity of a cultured pearl. Nacre thickness ranges from 0.1mm to over 2mm. Cultured pearls with nacre thickness over 0.4mm are highly recommended and they can last a life time. Pearls with thin nacre lack depth in their color and luster and the nacre can wear away soon. Skin chipping or peeling will eventually happen on these pearls. South Sea pearls have substantially more nacre than other types of pearls.
Nacre thickness is determined by the length of time the pearl is in the oyster. Nacre thickness can be accurately determined through x-rays. For pearl buyers, you can measure nacre thickness by looking at the pearl drill holes and see where the nucleus and nacre coating meet.
You can also examine nacre thickness by rolling the pearls around your finger back and force under a day light. If you can see the inner beads clearly or see peeled skin on the surface, the pearls have thin nacre.
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