Being stony means nothing, mate (4)

I believe the answer is:

opal

I'm a little stuck... Click here to teach me more about this clue!

'being stony means nothing mate' is the definition.
I can't tell whether this 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 "Gem stone" , "Semi-precious stone of b;uish white colour" , "Lustrous stone" , "Gem with delicate colours" , "Shimmering stone" .)

I've seen this clue in the Evening Standard.

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