One step from lapidary father to mother-of-pearl and the word is a little bit odd. I looked at the online etymology:
mother of pearl

1510, translating M.L. mater perlarum, with the first element perhaps connected in popular imagination with obsolete mother
a thick substance concreting in liquors; the lees or scum concreted" [Johnson], which is from the root of mud.

I really don't know how to read this line.

(it ís the shiny surface of the inside of oysters and shells, isn't it?)