It sound like she was using "cultured pearls" to mean cultured
freshwater pearls, and that that last dark drop was a dyed CFWP rather than a Tahitian.
The fact is, pearls are virtually all cultured now-- some with mantle tissue + beads (Akoya, SS, GSS, Tahitian, Cook Islands, Cortez, fireballs) or just mantle tissue (most freshwaters.) Not many natural pearls around anymore, and certainly not at the local jeweler's, though I know Jeremy sells some and maybe some other vendors here.
As for the "tooth test", it would not tell you whether the pearls were saltwater or freshwater-- only if they were real vs. imitation. Frankly I'd never do the tooth test on someone else's pearls-- it's so unsanitary; the human mouth carries so many bacteria. If I were the seller I wouldn't allow a customer to test my pearl with their tooth.
Instead I'd explain the other ways to determine if a pearl is imitation. Imitation pearls (and low luster cultured pearls) will not have overtones (Majorica imitation pearls have slight red and green fake orient but not overtones, which are seen in the dark central part of the pearl.) Lustrous white cultured pearls will show pink, silver, cream, green or blue overtones-- Tahitians have their own overtones. Tahitians do look different from dyed CFWP. In the dyed CFWP, the colors are more of an oil slick effect, or the way gasoline looks in a puddle-- a surface color, not a deep glow.
Also, if you examine the surface with a 10X loupe (magnifier) you will see that the surface of a cultured pearl looks smooth compared to that of an imitation pearl. To me the surface of an imitation pearl looks pebbly, like orange peel. If you have a loupe, you can practice with known cultured and imitation pearls. And if you don't already have a loupe-- it's a handy thing to own!
