Pearls arise because for the oyster and mussel, this is the only survival mechanism they have. 
For oysters and mussels, pearls are a weapon. Suppose during a delicious bite of plankton containing water a very small lobster enters the water, the oyster or mussel has no chance. So not because they have … right, Mother of Pearl. This substance is then used to render the intruder harmless and so the lobster immediately becomes the core of a pearl.
The three most important at the moment are:
Akoya; is the classic cultivated pearl and come in sizes from 5 to 11 mm. white pearls They are grown in Japan and China where prices above 9 mm can get very high. Colors range from cream to white-pink and to silver-gray. They also occur in golden yellow. The Akoya pearl in high quality stand out for their rare high lustre.
South Sea; is the most exclusive and most expensive type of cultivated pearl and come from the Pinctada Maxima oyster. In the warm seas off the Philippines and Australia, these oysters thrive best. They can range from 8 to 18 mm.Tahitian pearls are grown in shades of bluish-white to golden yellow.
Also South Sea pearl is a cultivé pearl from Tahiti from the pinctada Margaritifera. 
These are what are called black pearls and come in blue-gray to green-black and brownish-gold.
Freshwater; are grown in mussels and come mainly from China. Nowadays, good qualities are grown with core. This allows the pearl to be grown much colored pearls larger than without core, up to 15 mm. But also the coreless pearls are nowadays cultivated with excellent lusters, qualities that today more than ever can compete with pearls from oysters. They come in shades of white-pink, salmon orange to metallic purple.
Furthermore, we can distinguish the Keshi pearls, which can arise spontaneously during pearl 
cultivation in oysters. for example by an intruder such as a small lobster or a grain of sand. These are very whimsical to baroque and can also be drop-shaped. Highly sought after by enthusiasts. Shown here are silver-gray Tahiti keshi pearls (very rare).
On this site you can find even more information about the pearl.