Stone Age pottery, possibly first pieces found in county (8)
I believe the answer is:
sapphire
'stone' is the definition.
(I know that sapphire is a type of transparent gem)
'age pottery possibly first pieces found in county' is the wordplay.
'first pieces' indicates taking the first letters (I've seen 'first piece' mean this).
'found in' indicates putting letters inside.
'county' becomes 'shire' (I've seen this before).
The initial letters of 'age pottery possibly' is 'app'.
'app' put within 'shire' is 'SAPPHIRE'.
(Other definitions for sapphire that I've seen before include "Precious stone of rich blue" , "Deep blue - precious stone" , "Precious thing" , "Blue gemstone" , "Corundum gemstone" .)