This large natural Penn pearl was found off the gulf coast of Mexico and weighs an impressive 18.64 carats. 

Penn pearls are completely the byproduct of the mollusk fishing industry. They come from a shell called the Pinna rugosa or the Atrina maura. Only 30% of what is found are nacreous. 

If you're looking for something unique and one-of-a-kind, then this pearl is for you.