rachel fields natural pearls display sanibel shell festival

Natural Pearls with text by Rachel Fields…

(any text with * is my humble addition along with my photos)

What is a Natural Pearl?

A pearl is defined as a hard object, formed within the soft tissue of a living mollusk.  Usually pearl formation occurs within the mollusk in the organ called the mantle.  The mantle is normally responsible for secretion of the shell.  Pearls are usually made of calcium carbonate deposits which have been secreted in concentric layers.  A pearl is said to be natural when there was no human involvement in its formation.

rachel fields natural pearl exhibit close


Natural Pearl Formation

Natural (no human intervention) pearl formation occurs when a foreign object enters the soft tissue of a mollusk.  When the foreign material gets lodged near the mantle (the organ of the mollusk responsible for shell formation), the mollusk tries to protect itself by secreting layers of the same material used to create the shell.  It is rumored that the foreign material is often sand, however it is more likely a microbe or piece of shell (it might even be a piece of its own shell  if the mollusk gets deformed – see the deformed quahog shells in the display).

Mercenaria Quahog pearls


Bivalves vs. Gastropods

Both gastropods and bivalves produce natural pearls, however due to the shape, location of the mantle and increased difficulty of a foreign invader to enter the mantle, it is more rare for a gastropod to produce a pearl.

 bivalve versus vs gastropod mollusk

 Nacreous vs. Non-Nacreous Pearls

Pearls fall into one of two varieties, nacreous (*resembling Mother-of-Pearl) and non-nacreous.  Nacreous pearls are formed out of the material nacre and generally have high luster.   Mollusks that create nacreous pearls are usually bivalves and clams whose internal shells are made of mother of pearl.  Non-nacreous pearls are generally less valuable and durable with some exceptions, like some of the pearls displayed in this exhibit.

*In the next photos, the Abalone on the left makes NACREOUS PEARLS with an iridescent luster and the QUEEN CONCH makes gorgeous NON-NACREOUS pearls.

rachel fields natural pearl exhibit close    Strombus gigas queen conch pink pearls

Blister vs. Free-Form Pearls

Blister pearls are pearls that form flush against the shell of a mollusk forming a hemisphere that protrudes from the shell and is attached.  Free-form pearls are not attached to the shell.

*A blister pearl formed on a STIFF PEN SHELL (see the rest of the story of this BLISTER PEARL in a PEN SHELL- CLICK HERE)

Stiff Pen shell muscle scar with blister pearl Formed blister pearl in stiff pen shell


Flame structure, also referred to as chatoyancy, is a pattern that appears on many non-nacreous pearls. This pattern is the result of how the crystalline microstructure of the pearl interacts with penetrating light.

* This next photo is a 140-carat golden melo-melo pearl at the Phuket Sea Shell Museum we saw on a 2012 trip to Thailand. This museum houses one of the rarest melo-melo pearls in the world, known as the “Golden Pearl” perhaps because of its golden-yellow color and flame structure.

Melo Pearl from Baler shell

*Thank you Rachel, for providing such a fabulous exhibit for information and answers to so many questions we have about our beloved shells and what they are capable of making.


Pen shell Atrina rigida with Pearl  Pen shell pearl from Atrina rigida

Nodipecten subnodosus Lions Paw Pearls

rachel fields natural pearl exhibit