Thanks again emerald for your help and the ecellent link.
My heart is not rhodium plated, it is slightly tarnished as if it had been around awhile, it is heavy for its size, has the right packaging, and it is definitely not magnetic.
I sometimes need to test for gold or silver but I never before thought of the simple expedient of taking a little magnet around with me.
Magnets do not attract silver on copper and probably not some gold plated items, but they are a simple tool. A refrigerator magnet can be almost as small and flat as a business card.
Sounds like my heart is probably authentic. Good thing i don't plan to list it, though. The ebay penalties for making a mistake about Tiffany are severe, as they should be to protect the public. But researching some things can be inconclusive even if you make a lot of effort, as I am doing here.