I also have found many nuggets in my time detecting, but most are aluminum and you can tell right away. Although I have found 1 nugget, which SCREAMS silver, and has a weight as if it is silver. It is a nice dense nugget. It has a nice grey patina also. But I really am not sure what it is. It was found at an 1880's site. and was the only one. Someone said it could be "old silver solder" or something, but i haven't heard much about this. I guess the only thing left to do is test it sometime. Its never been a major priority though.