Pearls themselves are very unique. They are the only "gemstone" that is created by a living creature rather than by chemicals and pressure. For centuries they have been referred to as a gemstone by jewelry retailers. But they are not a gemstone at all. They are created when a mollusk (usually an oyster) gets a grain of sand inside of it's shell. This is an irritant to the mollusk and so the shell starts to grow around the sand in order to protect the resident oyster. A pearl is formed. Pearl Necklaces a so unique because you never find two that are identical. Pearl necklaces contain many pearls and each pearl all though it may be simliar in size, shape and color, it is very different. This has created much ado about pearl necklaces.
Finding all those pearls of similar size, color and shape takes lots of time. Pearls are not found in veins in the earth where one discovery could net 100's of carats like gemstones, but rather each pearl has to be removed from one single oyster by hand. Then they have to be sorted by hand until enough pearls of similar size and shape are gathered to make pearl necklaces. Gemstones can be cut to same size and shape which shortens the process. Pearl necklaces for this reason have a much higher human effort to create.