ummm.. not a sea pearl person. But I will have a go.
If a grain of sand gets into a clam it can cause damage and irritate so the clam coats it in lots of layers of protective material or shell (calcium-carbonate) and these build up over time into a round pearl.
Absolutely correct Craig (below) . This can occur naturally and they are the most expensive type or they can be cultured pearls where the whole thing is engineered in captured clams.
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