Get a magnet-- if your metal is attracted, there is iron or
stainless steel there
I have an anecdote. I don't know what it means-- but it happened.
I melted some casting buttons and poured them into water to make
shot. The resulting bits looked oxidized, so I had the brilliant
idea of throwing them into the tumbler. I figured I could just pick
up the steel shot with a magnet and leave the silver. It worked, but
took for-e-ver!! Anyway, a few of the pieces of silver stuck to the
magnet! I removed any that did, but there is no apparent way the
silver could have gotten contaminated with iron or nickel or
anything else. I have no idea why this occurred.