Pearl's mother (5)
I believe the answer is:
nacre
This clue is a double definition.
'pearl's' is the first definition.
(nacre is another name for mother-of-pearl)
'mother' is the second definition.
(I have seen 'Mother of pearl ' mean 'nacre' so perhaps 'mother' could also mean 'nacre')
(Other definitions for nacre that I've seen before include "Mother-of-pearl - crane (anag)" , "Mother-of-pearl from shellfish" , "Constituent of inlay furniture" , "inside of shell" , "valuable material" .)