Working coordinates of precious stone (4)
I believe the answer is:
'precious stone' is the definition.
(I know that onyx is a type of precious stone)
'working coordinates' is the wordplay.
'working' becomes 'on' (e.g. a machine is working or on).
'coordinates' becomes 'yx' (I can't justify this - if you can you should believe this answer much more).
'of' is the link.
Can you help me to learn more?
(Other definitions for onyx that I've seen before include "Stone-like marble with different colours in layers" , "Banded chalcedony" , "Black and white striped gemstone" , "Striped agate" , "Gemstone, a variety of chalcedony" .)