Are basalt rocks gypsum? (9)
I believe the answer is:
alabaster
'gypsum?' is the definition.
(thesaurus)
'are basalt rocks' is the wordplay.
'rocks' indicates anagramming the letters.
'are'+'basalt'='arebasalt'
'arebasalt' anagrammed gives 'ALABASTER'.
(Other definitions for alabaster that I've seen before include "Translucent white gypsum as found carved into ornaments" , "Stab a real odd white mineral" , "White mineral used for carving" , "Arabs tale (anag.)" , "Type of gypsum" .)