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Body parts fossils were formed when a plant or animal died and was buried quickly in wet sediment. The soft body parts rotted. This left hard parts like bones, teeth, or shells. The weight of the mud layers in which they were buried pressed down, causing minerals in the ground water to seep into the cell spaces of the hard parts. Eventually, the bones or teeth or shells were replaced by these minerals, and they became a rock. Silica, calcite, and pyrite are common minerals involved in the process of making body parts into fossils. These fossils often take on the color of the minerals from which they were made. This process of making fossils is called permineralization.