Roger,
I am aware of the OP's question which is not related to your post to which I was commenting.
Other than some moral or ethical rationale, the law here does not care what the value of the object is. You are not legally compelled to report the finding. This is essentially one of the problems individuals have trying to make claims to things of value that they have lost but whose ownership they cannot prove.
Years ago I bought a computer for my daughter to use in college. Sometime thereafter her keyboard requied a replacement. She took it in for service only to be told that the computer's serial number was reported as belonging to a computer stolen from Dell. Being a good citizen I contacted Dell who essentially refused to discuss it with me. They would neither admit nor deny the computer was stolen, but the serial number was on A list (wouldn't say what that list was.) I found this all strange, but until I cleared up the matter I could not order my daughter a replacement keyboard from them.
Still trying to be a good citizen I asked my daughter to contact the police to see if the computer was on the National Hot List. It wasn't. I asked the police for advise as to I should do. They said keep it. If there was a real owner somewhere who made a provable claim I would have to return it or the money equivalent. So that's what my daughter did. She used it a couple more years then sold it to one of her friends. Never heard anything more about it from any person or from Dell.
Now, of course the computer wouldn't be worth a diamond lying on the street (if it was pretty big, that is.) Still, the story played out as I suspected it would. Even the police later said she could have saved time by not even bothering to report it.