When an irritant, such as sand or a parasite, makes its way between the oyster’s shell and its mantle (the organ that creates the shell), the oyster’s body jumps into defense mode. The mantle starts to wall off the foreign object with nacre, the same substance used in the inner shell and what most would call “mother of pearl.” This process creates a pearl that gets bigger as time goes on, and it stays within the oyster until the oyster dies or humans harvest the pearl.
But not all pearls are the near-perfect spheres you see on necklaces; rather, the pearl takes the shape of whatever object it covers. And different breeds of oyster create different colors of pearls. That’s why you’ll find pearls in oval, teardrop, and “baroque” shapes, and colors ranging from bright white to pale pink to deep black.