aragonite

noun

1. (countable, uncountable)An evaporite consisting of anhydrous calcium carbonate with the chemical formula CaCO₃ and occurring in pearls, shells and nacre; it is dimorphous with calcite.

   “The basic chemical material of molluscan shells is calcium carbonate, which forms the outer layer of calcite, and the inner layer of aragonite, which is a heavier and harder substance although it has the same chemical composition.”