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.
akoya pearls can cost several times more than freshwater pearls of similar quality and size one reason for this it that one mussel in a lake can produce as many as 40 freshwater pearls in one harvest season an oyster in the sea usually produces one or two saltwater pearls at a time