Pearls are gemstones, too!
A pearl is a solid round gem made of Mother-of-pearl. It forms in certain shells and sometimes in snails. Why and how this happens in the molluscs, is still unexplained.
Calcium carbonate or aragonite forms about 80 – 92 % of a Pearl, additionally, calcium carbonate in its secondary form, calcite, and a small amount of water are included. They are very hard and can withstand impact with a Mohs hardness of 3.5 to 4.
The Pearl’s beautiful luster is created by reflection and refraction of light at the crystal lattice edges where there are water molecules.