Being stony is nothing, mate (4)
I believe the answer is:
opal
'being stony is' is the definition.
I can't judge whether this definition defines the answer.
'nothing mate' is the wordplay.
'nothing' becomes 'o' (looks like zero - 0).
'mate' becomes 'pal' (I've seen this before**).
'o'+'pal'='OPAL'
Can you help me to learn more?
(Other definitions for opal that I've seen before include "Semi-precious stone of milky or bluish colour" , "Something milky-white" , "Milky-white gem" , "Shimmering stone" , "Semi-transparent gem" .)