First, you have to know the kind of pearl that is available in the market. They are categorized in two:
1. Saltwater pearls - these are formed inside oysters that are living in the heart of the ocean. It has three types - Tahitian, Akoya, and South Sea Pearls - depending on the kind of oyster that produced it.
2. Freshwater pearls - these are formed inside mussels that are living in lakes, rivers, ponds, or any other body of fresh water.
Either saltwater pearls or fresh water pearls can be natural or cultured. Natural means that there is no human intervention and the pearl is formed naturally by the oyster or mussel. The pearl oyster secretes nacre an iridescent material made of calcium carbonate. The nacre will coat a microscopic irritant entering its shell until a pearl is formed. On the other hand, cultured means that a pearl grower will carefully open the shell and introduce a small object as an irritant (commonly a bead or metal), then get them back into the water and wait for the pearl to form. This usually takes 2-5 years.
There are a lot of fake pearls out there, but it is more pleasant to own a genuine pearl jewelry at least once in your lifetime. A genuine pearl does not only mean "natural". Even "cultured" pearls are genuine pearls too, having an advantage of being more affordable than natural pearls.
When you hear of fake pearls, it means that they are man-made pearls made of glass, ceramics, plastics or any other similar materials. They are also called "faux" pearls and are sold as costume jewelry. In the face of modern technology, man can create pearls that look like the original one. Mind you, fakes can have the exact original look, but they do not have the same weight or texture. And their luster is dimmer, which is enough to show that they are imitations of the original. They do not contain any gem value.
To help you with identifying genuine fresh water pearls, here is a three way test:
1. Tooth test - get the pearl and run it against your front teeth, just right below the biting edge. If you sense a gritty feeling, it is most probably real. Fake pearls are smooth, like plastic or glass. However, be very careful of the pearls that are made from ground shells as they can also be gritty.
2. Luster test - test the pearls against all kinds of light. Fake pearls will have a shine only on the surface, while genuine pearls always shine from within.
3. Rub test - get two pearls and rub it together. It should be gritty and do not slide off each other.