Why Are Pearls So Expensive?

Natural pearls are rare, because they don't really happen organically. Pearls are an oyster or a mollusk's way of dealing with an irritant, which has found its way into its shell (so yes something has to fall into the bivalve). To ensure that it is protected from this irritant, which could be a small organism, the oyster or mussel starts coating the intruder with nacre, which is the same shiny material found on the inside of its shell (this nacre is also known as mother-of-pearl). By throwing on layers of nacre around this irritant, the oyster (or mollusk) creates a pearl (via Business Insider). Of the approximately 20,000 different types of bivalves, only a handful can make pearls (via Pearl Guide).

Natural pearls and cultured pearls are different because of the way the irritant is introduced. A natural pearl is formed when an irritant makes its way into an oyster, while a cultured pearl is made when the irritant is planted into the oyster by a human. Business Insider says the irritant isn't just any old piece of dirt — it is normally a piece of mollusk tissue (also known as donor tissue), so the oyster's defense response is triggered, forcing the mollusk to create a pearl.