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.
Saltwater pearls include the akoya cultured pearls grown in Japanese and Chinese waters. They range in size from the tiny 2mm to the rare 10mm and are usually white or cream in color and round in shape. Australia, Indonesia, and the Philippines produce the South Sea pearl - the largest of all the pearls.