Final answer:
A cast fossil is made of rock and is a replica of the original bone or shell, which was made of organic material.
Step-by-step explanation:
A cast fossil is different from an original bone or shell because it is made of rock, while the original is made of organic material. The process of fossilization begins when an organism dies and is buried in sediment. Over time, the organic material in the bone or shell can be replaced by minerals, creating a fossil that is a replica of the original, but now made of rock.
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