These are formed in natural pearl oysters that live in tropical oceans and are usually cultivated in protected lagoons.  The three main types are Akoya, South Sea and Tahitian.  In order to ensure a more round pearl only a small part of the oyster is used which typically makes just one or two pearls at a time.  Some pearls are re-seeded to create larger pearls over time, but much effort is spent in grading each finished pearl to match up as near as possible in size, shape, lustre and brilliance for a string necklace.    ..........picture shows a Pearl Oyster Farm in Seram, Indonesia.