@ Alchemy- Freshwater pearls are different from saltwater pearls. If you are purchasing a black pearl necklace, you should probably go with a high quality cultured pearl necklace versus a natural pearl necklace because the cost will be very significant, and you will not be able to tell the difference without very expensive machinery.
To tell the difference between cultured and natural pearls you need to use a gemological x-ray. natural pearls have concentric rings that go all the way to the core of the pearl, while a cultured pearl is characterized by a starter bead, or a piece of mantle. Beaded and beadless cultured pearls will both have very different core x-rays from a natural pearl that started forming mother of pearl rings around a microbial parasite or piece of organic matter.
If you need to tell the difference between an imitation pearl and a cultured or natural pearl, you can use the tooth test. If you rub a pearl against a tooth or another pearl, and it feels silky smooth, you have a fake. Real pearls have tiny bumps and divots.