Rock forms are a blast (9)
I believe the answer is:
alabaster
'rock' is the definition.
The definition and answer can be both substances as well as being singular nouns.
Perhaps they are linked in a way I don't understand?
'forms are a blast' is the wordplay.
'forms' indicates an anagram.
'are a' becomes 'area' (maths abbreviation).
'area'+'blast'='areablast'
'areablast' anagrammed gives 'ALABASTER'.
Can you help me to learn more?
(Other definitions for alabaster that I've seen before include "Type of gypsum" , "Stone often used for carving" , "Material for sculpting" , "medium of sculpture" , "Altar base (anag)" .)