Pearls are certainly beautiful, and parasites are not. So, how does a parasite become a priceless pearl? First, a parasite enters the shell of a mollusk. The now-irritated mollusk forms a sac that encases the parasite; the sac is made of a substance called nacre. The mollusk repeats the process of forming the sac and encasing the parasite and the sac several times. This ball of nacre begins to harden. A few years later, voila! A pearl!