Pearls are the only gemstones created by a living thing
Pearls are formed whan an irritant, such as a grain of sand, enters the shell of an oyster. The oyster secretes a substance called nacre to wall off the irritant. This nacre gradually builds up over time until a beautiful pearls is formed.
Cultured Pearls are not man made or synthetic. They are formed with the help of man; a small bead ( the nucleate) is placed inside the oyster shell, around which the nacre grows. Cultured pearls tend to me almost perfectly round in shape.
Freshwater pearls come not from oysters, which are sea creatures, but from mussels. Freshwater pearls can also be cultured, but the nucleate is not a round bead; it is a tiny pice of tissue from a donor mussel. Unlike cultured oyster pearls, which retain the tiny nucleating bead inside them, cultured freshwater pearls are all nacre.
Because the nucleate for freshwater pearls is mussel tissue, the pearls will never be perfectly round; the most common shape is button (60%) followed by baroque and semi-baroque (38%)
Imitation or faux ("false") pearls can look extremely good, and some are very well made. It is very easy to tell the difference between a genuine pearl and a false one; simply put the pearl very gently between your teeth and rub your teet very gently across the surface. The genuine pearl will feel gritty whilst the imitation pearls feels smooth.