Natural Shell Blisters and Blister Pearls: What’s the Difference? GIA Report

The majority of saltwater and freshwater pearls bought and sold in the global jewelry markets are “whole pearls,” sometimes referred to as “cyst pearls” (CIBJO, 2017). These pearls form within the soft tissue of a mollusk, either naturally or with human intervention as complete pearls, hence the term “whole.” Such pearls are usually more desirable and command higher prices than those classified as “blister pearls.” While not technically a pearl, there is a third type of formation known as a “shell blister” or “blister” (Strack, 2006) that needs to be considered when addressing this subject. This article addresses the differences between these three materials, with a focus on natural shell blisters and natural blister pearls (figure 1). In order to fully understand these two organic formations, it is helpful to first discuss a natural cyst (whole) pearl in more detail.

A natural cyst (whole) pearl starts forming within a mollusk’s soft tissue and remains in the pearl sac until the time that it is removed (figure 2). While some gemologists believe that this definition is the only one that should be applied to natural cyst pearls, others consider any natural pearl that breaks through the mantle and starts to transition into a natural blister pearl, even when not covered with enough shell layers to make the pearl and shell one entity a natural cyst pearl. The latter type is sometimes found on a shell’s inner surface and may be freed with sufficient pressure from the fingers. No form of cutting or grinding is required in such cases.

A natural blister pearl forms naturally as a whole pearl within a pearl sac in the mantle (see natural cyst pearl). The natural pearl breaks through the sac and eventually finds its way between the shell and mantle. Nacreous or non-nacreous layers of calcium carbonate are secreted over the pearl and attach it to the shell (CIBJO, 2017). The completely merged entity is now by definition a natural blister pearl (figure 3). As the mollusk grows older, the thickness of the enveloping layers increases. Unlike whole pearls, these are removed from their shells through cutting and grinding in order to be suitable for use in jewelry as loose pearls. Some smaller shells with blister pearls are incorporated in pieces of jewelry with unique designs.

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