Akoya pearls are the proper term for Japanese saltwater cultured pearls, which were the very first pearls to be grown inside Akoya oysters using a process involving human interference and skill. Kōkichi Mikimoto was the first entrepreneur to commercialize this process, which was patented in 1916 by his son-in-law. The process involves the precise, surgical placement of an irritant inside the soft tissue of the oyster, which as part of a natural process, ends up secreting calcium carbonate and conchiolin to cover the irritant. Layers and layers later, a few of these oysters produce an amazing, lustrous Akoya pearl. Depending on the size of the oyster, they grow from 3-10mm. The grading and valuing of pearls is a more complex process than doing the same for diamonds.
Little known tidbit: the heart of the vast majority of saltwater pearls is a small sphere of mollusk shell, or “mother of pearl,” taken from mussels that come from the Mississippi River. No matter how far away the pearls are cultured, most are "made in the US" at their core.