I found this in a local park and it was fairly deep. I would suspect that it might be 40+ years old. The park is so trashy that I did several pull tabs out of each hole I decide to dig. This hole was no exception.
Since it is black, it might be sterling. Look for a mark. If you want to test it, take a piece of aluminum foil, spit on it to get it all wet, sprinkle a tiny amount of salt, mix it all around, and fold the foil very tightly over the piece. If it is silver, it will smell like sulphur and may even get hot. It will do a good job of cleaning it.
Guardian angel pendant. St. Michael, an Archangel, defeating the Beast. I found one of these a couple weeks ago...mine is brass, marked Italy. Yours appears silver or silver plate, nice find. Niffler