ELI5 Pearls - how does a grain of sand make such treasure?

What makes a pearl so mesmerizing, and what is it really? What exactly is a pearl made of, and how does it form inside a mollusk? Can a grain of sand truly cause a perfect pearl? What's the difference between a natural pearl and the cultured ones we see today? How do pearls get their stunning array of colors, from soft pinks to deep blacks? What makes certain pearls incredibly valuable, fetching millions?

... we explain like I'm five

Thank you to the r/explainlikeimfive community and in particular the following users whose questions and comments formed the basis of this discussion: emoposer, iamkunii, cooldrummer1208, Albres, discordant_rhyme, xentonian, enzio901, n19h7m4r3

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