Doctor, it's nicer varieties of yellow quartz (8)

I believe the answer is:

citrines

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'varieties of yellow quartz' is the definition.
Although both the answer and definition are plural nouns, I don't understand how one could define the other.

'doctor it's nicer' is the wordplay.
'doctor' is an anagram indicator.
'its'+'nicer'='itsnicer'
'itsnicer' is an anagram of 'CITRINES'.

Can you help me to learn more?

I've seen this clue in the Sydney Morning Herald.

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