This looks interesting... So you say that you weren't tempted to check it any further all this time? I assume you must've already checked if a magnet catches on it (silver is non-magnetic)? Other than that you could perhaps also try doing a water density test, or confirm the following:

Streak test: rub it gently on the back of an unglazed ceramic tile. Silver should leave a shiny grey streak while most regular rocks would leave white-ish/brown-ish.
Sound test: Silver produces a high-pitched ring when struck.

I won't recommend you doing the aluminum foil test mentioned above just yet, just in case that's some valueable ancient artifact you've actually stumbled upon 😁