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Teeth from living sharks are white, but fossilized shark teeth are usually black. Why?

Answer:

Shark teeth are made up of calcium phosphate, and the tooth has many tiny pores. When the tooth is buried, ground water carrying minerals can be pulled into those pores, much like water into a sponge. The minerals are deposited, giving the tooth its color. The black and gray colors come from manganese dioxide. Some teeth are brown, colored by iron, although I have found shark teeth near Waurika, Oklahoma that were almost purple due to iron.
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