It does resemble a mollusk shell. It looks like a weathered dead animal on the outside, with all the periostracum completely worn away, but the nacre in the picture would indicate a more recent dead animal because the nacre still appears to have a deep shiny finish. I would have to get better pictures and good idea of scale. Are you sure it is one solid piece or does it hinge and bend? Because if it is a bivalve and the valves were closed, protecting the nacre over most of the weathering period, it would not necessarily be uncommon to have a better preserved shiny nacre. If it is one solid piece then this probably is not the case as the nacre would likely have been exposed to the elements the same as the outside of the shell. But not necessarily, if it were a snail species and inverted (meaning the inside was laying down in the mud with the outside facing the elements) then we could still see a similar scenario. Especially if it was found on the surface and out of the water in an area that was say more likely to experience acid rain. Though I think this would be less likely.
Is the outside soft and chalky? Is the inside smooth and slick? Was it found in or near a natural water source? Were there other similar pieces in the area? Did you dig this or find it laying on the surface? Do I ask too many questions? :O)
Why did you post in this forum? Was this found in a known Native American artifact area? I ask because you do not indicate where you live and it may be a piece of a shell midden. Again unlikely as one never finds really nice nacre in old shell midden.