Why are sharks teeth black that you find?

Shark teeth are made up of calcium phosphate, which is the mineral apatite. Although shark teeth are sturdier than the cartilage that makes up their skeleton, the teeth still disintegrate over time unless they are fossilized. This is why you rarely find white shark teeth on a beach.

Shark teeth are preserved if the tooth is buried, which prevents decomposition by oxygen and bacteria. Shark teeth buried in sediments absorb surrounding minerals, turning them from a normal whitish tooth color to a deeper color, usually black, gray or tan. The fossilization process takes at least 10,000 years, although some fossil shark's teeth are millions of years old! Fossils are old, but you can't tell the approximate age of a shark tooth simply by its color because the color (black, gray, brown) depends completely on the chemical composition of the sediment that replaced the calcium during the fossilization process.