I'm learning... Mel Fischers treasure ship Atocha(?) with the buckets of pearls keeps nagging me for some reason, all the pearls that survived for centuries - in saltwater, though. Maybe someone can tell us if tannic acid would eat pearls? Any vegetable matter decomposing in the reservoir could produce tannic acid and even if not highly concentrated like swamps and glades, maybe a necklace buried in the vegetation would be in a highly acidic squish.
I love that region. Some of the wells in the finger lakes region in NY state have a sulphurous smell and taste. You can't discount other mineral contents, iron for one; there are all sorts of springs there.
Why not clean em up, then give them a scrape? This is just for fun, the 'pros knows', and the cheap clasp generally points to inexpensive pearls, if they were pearls. Inexpensive pearls are still enjoyable, if they clean up nicely for you.
Fakes can be fun, but they can wear out pretty fast if not well made. Some fakes, like older Majoricas can be pretty and durable with good care, but pearls are our passion, so...
One other thing, some stones (quartz-like, etc) also feel gritty rubbed across teeth.