Typical albuminoids are gelatin, keratin, elastin, fibroin, spongin and conchiolin.
" Conchiolin," the matrix of shells of the mollusca, is only slightly soluble in acids.
The shell consists of an organic basis the substance of which is called conchiolin, impregnated with carbonate of lime, with a small proportion, I-2%, of phosphate of lime.
On the outside of the shell is a non-calcified layer of conchiolin called the periostracum, secreted by the thickened edge of the mantle.
Nacre is made of two substances, aragonite -- which is translucent, giving the pearl its beautiful lustrous appearance and conchiolin -- which acts as a glue holding the layers of aragonite together.