There is a very simple way to tell if a pearl is genuine - if you rub it gently across the front of your teeth - not the edge, but the flat part of the tooth - a genuine pearl will feel 'gritty', a fake one will feel smooth. There are very very few 'natural' pearls available anymore and natural pearls will cost many thousands of dollars. If someone tells you they have a 'natural' pearl to sell you, request an xray and certificate of authenticity to show that there is no bead inside the pearl, which is the 'seed' that is put in cultured pearls. Cultured pearls are considered 'real' pearls. Whereas natural pearls are grown from a grain of sand in an oyster, cultured pearls have 'seeds' placed in the oyster and the pearls are grown from those.
Here is an excellent site to help you with quality and pricing of pearls. Hopefully this will help !!