Humans have always been amazed with the glinting, sparkling minerals that our Earth produces. But do all shining jewels originate underground? These objects do mostly come from geological processes and sedimentary rock. As National Geographic describes, they form when either lava or magma cools down, or when mineral-rich water is trapped.
But there is one precious gem, as noted by The Smithsonian Magazine, that is made by a living creature, and it is certainly prized very highly. Oyster pearls, both freshwater and saltwater, come from the reaction of this bi-valve mollusk to irritation under the shell.
How Do Oysters Make Pearls?
Oyster pearls are the result of irritation. The shell is set up as an outer layer to protect the soft body of the oyster. Between the oyster and its shell lies the mantle, described by How Stuff Works as the organ that secretes the components of the shell. The mantle creates shell out of minerals consumed by the oyster as food. This is an ongoing process that allows for growth, and also repair if the shell is damaged.
The inner layer of shell sits right next to the mantle. It’s a smooth, shiny layer made of calcium carbonate, conchiolin and aragonite, also known as nacre. Another name for this is mother-of-pearl, and this is in fact where a pearl forms.
When an irritant, like a fragment of shell or sand, gets between the mantle and the shell, the oyster starts covering it up to reduce the itch, so to speak. According to The Natural History Museum, the mantle secretes nacre around the irritant as a natural defense, building up layer upon smooth layer, with a beautiful pearl as the end result.
Do All Shellfish Make Pearls?
Although clams and mussels can also make pearls, not all shellfish secrete nacre from their mantle. Pearl farmers choose species of oyster that are known to be good at growing pearls. These will either be harvested from natural growth or cultivated by stimulating the oyster directly to form a pearl. When growing artificial pearls, also known as cultured pearls, farmers may select certain oyster species that will form pearls in either freshwater or seawater. Cultured pearls still make pearls in response to an irritant, but in this case, the farmer introduces the irritant into a cut made between the mantle and the shell.
According to American Pearl, pearl formation can take between six months and four years. Farmers harvest oyster pearls carefully through a small incision and may use an oyster repeatedly for harvesting pearls. Harvesting doesn’t necessarily lead to death for the oyster.
Pearls come in several colors — white, black, gray, green, blue and red. They can be round and smooth, or irregular, as in baroque pearls. Color is variable, but only oysters in the South Pacific create black pearls.
Ocean Biominerals and Climate Change
Understanding how oysters make pearls can provide insights to support climate change research. Global warming is taking place because of rising levels of greenhouse gases in the Earth’s atmosphere, and as these gases increase in abundance, more of the Sun’s energy is trapped. Accordingly, researchers have noted increases in ocean temperatures.
Not only are oceans getting warmer, but they’re also trapping increasing amounts of carbon dioxide, which is making the water more acidic. This is affecting the formation of biominerals like coral, mollusk shells and maybe even pearls.
Since shells are made of carbonate materials, rising acidity is dissolving them and making them thinner. This affects the ability of the shellfish to survive and multiply. Biominerals like corals and mollusk shells are showing evidence of climate change, and according to The Smithsonian Magazine, they’re also great records of climate change in the past. Ancient bacterial colonies called stromatolites and corals grow in layers, each one affected by surrounding conditions. These rings of growth provide a visual record for the conditions in which the organism grew, granting researchers a glimpse into how our planet adapted to periods of climate change in the past.
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