How are pearls formed? 

Pearls are the result of a biological process within living sea creatures like an oyster as it protects itself from foreign substances. An oyster has two shells. Inside the shells, there is an organ called mantle. Mantle covers and protects all the internal organs of the oyster. Whenever any foreign substance like a grain of sand or a parasite makes its way into the oyster and gets stuck in between the shell and mantle, the mantle’s natural reaction is to cover that foreign substance to protect the oyster's internal organs. For this, the mantle secretes a mineral called nacre around the foreign substance in layers. Over time, layer upon layer of nacre leads to the formation of a pearl.