Let's say you happened to make a transaction with someone who you didn't know to be a criminal. He buys a nice ring for the girl on his arm and gives you $1K in marked bills (which are bills that have serial numbers recorded by some agency).
Now in that case, he's flashing around cash, which is not that uncommon say in certain locales (Las Vegas comes to mind). In that case, the cash you received doesn't lose any value. Though you may get questioned by the police.
Now ink stained bills and bills glued to each other should be rejected since they were likely from a bank or atm heist. But that makes it easy to spot.
Well diamonds are fungible for any definition of the word you may choose and for the vast majority of the stones. Some of them are unique in that they're very large, very pure, have been worked on so much they are identifiable, have an IAG number engraved, etc. but the vast, vast majority of diamonds you can trade at any diamond shop in a very fungible manner, no questions asked.
