system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why are shark teeth I find on the beach always black, but when they are in a living shark they are white? Why are there no white shark teeth on the beach?

Along the same line, why are they always white in a shark jaw in a museum or mounted on a fisherman's wall, but not on the beach? Makes me think it doesn't strictly have to do with whether they are alive or not.

Edit: thanks for all the responses. I had no idea I was finding fossils. I always thought fossil finding was hard and required digging. So much cooler knowing I find and hold things that are thousands of years old!
gpt
The teeth you find on the beach are fossilized

> 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.