Layers of calcium carbonate , or laminated aragonite.

nacre is the substance that covers a pearl naturally as it is forming

I believe it is is called nacre. I read that nacre is what pearls are formed from.

"Nacre" is not a type of pearl, but rather the material coating that creates a pearl.

A pearl is made up of layers and layers of a substance called nacre -- calcium carbonate and protein. The thicker the nacre is, the more durable and valuable the pearl. The average thickness of a cultured pearl's nacre ranges from 1mm to 3mm.

"Nacre" is a pearly substance that mollusks secrete around irritants. Thick, overlapping nacre layers create a strong luster and a valuable pearl. Thick nacre layers take years to build up. It can be used to determine whether a pearl is real.

nacre

Macle - nacre is the lining of a shell