One of The Stones in pre-eminent area and zone (5)
I believe the answer is:
topaz
'one of the stones in' is the definition.
The definition and answer can be both substances as well as being singular nouns.
Maybe you can see an association between them that I can't see?
'pre-eminent area and zone' is the wordplay.
'pre-eminent' becomes 'top' (I can't justify this - if you can you should give a lot more credence to this answer).
'area' becomes 'a' (maths abbreviation).
'and' means one lot of letters go next to another.
'zone' becomes 'z'.
'top'+'a'+'z'='TOPAZ'
Can you help me to learn more?
(Other definitions for topaz that I've seen before include "Yellow or pale blue gem" , "valuable item" , "Shade of dark yellow" , "Shade of yellow -- gem" , "White or colourless gemstone" .)