Work on the final piece of stone (4)
I believe the answer is:
opal
'stone' is the definition.
(gemstone)
'work on the final piece' is the wordplay.
'work' becomes 'op' (abbreviation for opus).
'on' means one lot of letters go next to another (in a down clue, letters appear on others).
'the final piece' becomes 'al' (I can't justify this - if you can you should believe this answer much more).
'op'+'al'='OPAL'
'of' acts as a link.
Can you help me to learn more?
(Other definitions for opal that I've seen before include "Lustrous stone" , "Rock" , "Opaque gem" , "Semi-precious stone of b;uish white colour" , "often semi-transparent" .)