Pearl Cross Pendant What Is The Difference Between Mother Of Pearl And Shell?

What is the difference between mother of pearl and shell? - pearl cross pendant

Hello, I'm curious, I want to know is the differnce between the nacre and the shell? I have a pendant cross pendant shell and made a big round of nacre as the one who sold me them to me (different provider) told you, but they look like? So what's the difference, if so, how? Thank you.