Pearl's mum is grey but beautiful (5)
I believe the answer is:
nacre
'pearl's mum' is the definition.
(I've seen this before)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for nacre that I've seen before include "inside of shell" , "Mother of pearl" , "Mother-of-pearl - crane (anag)" , "Constituent of inlay furniture" , "valuable material" .)