If you’ve ever played 20 Questions, you’ve probably come across the opening phrase: animal, plant, or mineral? But which category would you put a pearl in? Pearls may be the queen of gems and the gem of queens according to Grace Kelly, but what are they technically classified as? Unlike the average gemstone found naturally in earthen deposits, these tiny spheres are crafted deep in the ocean by a range of shelled sea creatures.
How are pearls formed?
Pearling occurs when an object—be it a grain of sand, a particle of food, or a parasite—enters the shell of a mollusk. Technically, clams, mussels, and oysters can all make pearls, but the most common types are made by oysters. Oysters and clams have very soft bodies, so they need to protect themselves from sharp, irritating objects that get in their way. To do this, they cover the object with layers of calcium carbonate in two different forms.
“Pearl is a word we use for a shiny creation that produces a mollusk. When debris gets stuck in a mollusk and they can’t flush it out, they coat that debris with their own nacreous or shell material,” said Gabriela Farfan, environmental mineralologist and Coralyn W. Whitney curator of Gems and Minerals at the Smithsonian’s National Museum of Natural History , in Smithsonian Magazine.
What are pearls made of?
Pearls are therefore composed of the original object that irritated the mollusc, and then layers of various forms of calcium carbonate along with a substance called conchiolin. Conchiolin is a type of protein that holds together the layers of calcium carbonate inside the pearl.
There are usually two forms of calcium carbonate in a bead. The interior of the pearl consists of columnar calcite or aragonite. As the pearl grows, the outer layers transform into tabular aragonite, or nacre, also known as nacre. This is the same substance often found inside seashells and other shells that gives pearls their famous iridescent luster.
Oysters and clams can produce pearls naturally, but most commercially available pearls are made on oyster farms, where the initial irritating object is a small plastic or glass seed placed inside intentionally farmed oysters.
Are pearls minerals?
Minerals are naturally occurring solid inorganic substances with a specific chemical composition and ordered internal structure. Pearls are not considered minerals because their internal structure does not have organized crystals.
Aragonite and Calcite, which occur naturally in the earth, have the required crystalline structure and are therefore considered minerals. However, the main reason pearls are not considered minerals is the conchiolin – this material is made up of protein and while essential to the formation of the pearl is a biological material and therefore means these beautiful lustrous pearls are not minerals.