Mother of Pearl Tooth Rare Genuine 12/13mm

This is something I have never heard of before an acquaintance pointed it out to me. I thought it was just a strange but huge pearl. I'm relatively sure this is saltwater. Ofcourse I don't know but I would guess the clam this came from would've been able to produce a 10mm pearl. What IS a Mother of Pearl tooth? As opposed to how a Pearl forms - as a defensive response to something like a grain of sand inside the clam - this one grew from the Mother of Pearl part. It's an extremely rare phenomenon. Basically, instead of a foreign object entering the clam, in this case something else caused it to essentially form a 'growth' as we would see it. This 'tooth' was rooted in the Mother of Pearl rather than being formed inside due to the previously mentioned grain of sand.