Above is seen a 5mm pearl purchased as an Akoya cultured pearl. Notice that it is uniformly round from the seed all the way out to the outer nacre layer, which is the thin line you see at the very edge of the pearl. What was quite remarkable about this pearl is that the center of the pearl is a hollow plastic looking bead, not the Mississippi mussel shell bead that was expected. You can see the circular structure of the Akoya cultured pearl with the bead and resulting layers being rather uniform from center to outer nacre layer.
The center is a plastic bead. Apparently someone has found out how to use plastic instead of natural material to get the Pearl to form.
And taking it one step further, perhaps a reused piece of plastic for the starter bead. Why the multiple holes in the starter bead? Was the pearl farmer recycling plastic? Why place a used, multi-drilled plastic starter bead inside an oyster to make what could turn out to be a very expensive South Sea Tahitian Pearl in the first place?
But then again, it is this type of perplexing problems that is what makes gemology so much fun and of interesting experiences. A lifetime of learning and sharing the wonders of the earth.
Again thanks to the ISG for the use of pictures and information.