This shell layer is composed of a type of protein known as conchiolin. Corneous opercula are made out of the protein conchiolin. Chemically speaking, this is calcium carbonate and a fibrous protein called conchiolin. Both genera are characterised by partially uncalcified (conchiolin) shell portions. Natural pearls are nearly 100% calcium carbonate and conchiolin. There are no visible growth lines of conchiolin in cross-sections of sclerites. The combination of aragonite and conchiolin is called nacre, which makes up mother-of-pearl. Conchiolin is composed largely of quinone-tanned proteins.