There are two ways to remove coal from the ground: surface mining and underground mining. Surface mining is used when a coal seam is relatively close to the surface, usually within 200 feet. The first step in surface mining is to remove and store the soil and rock covering the coal (called the overburden).
Pearl oysters are not closely related to true oysters, being members of a distinct family, the feathered oysters (Pteriidae). Both cultured pearls and natural pearls can be extracted from pearl oysters, though other molluscs, such as the freshwater mussels, also yield pearls of commercial value.
How Do Mollusks Make Pearls? Pearls are formed when an irritant, such as a bit of food, grain of sand, bacteria or even a piece of the mollusk's mantle becomes trapped in the mollusk. To protect itself, the mollusk secretes the substances aragonite (a mineral) and conchiolin (a protein), which are the same substances it secretes to form its shell.