Crossword Genius

Stone axes have zero rebound (4)

Ross

I believe the answer is:

onyx

'stone' is the definition.
(type of gemstone)

'axes have zero rebound' is the wordplay.
'axes' becomes 'xy' (x axis and y axis).
'have' is a charade indicator (letters next to each other).
'zero' becomes 'no' (similar in meaning).
'rebound' says the letters should be written in reverse (I've seen 'rebounds' mean this).
'xy'+'no'='xyno'
'xyno' reversed gives 'ONYX'.

(Other definitions for onyx that I've seen before include "Stone like marble with layered colours" , "Banded gemstone" , "Kind of agate" , "Black variety of quartz" , "Marble-like gemstone" .)

I've seen this clue in The Guardian.
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