A gem of a green (7)

I believe the answer is:

emerald

I believe this clue is a double definition.

'a gem' is the first definition.
(type of gem stone)

'a green' is the second definition.
(emerald is a shade of green)

'of' acts as a link.

(Other definitions for emerald that I've seen before include "Precious green stone" , "Green gemstone" , "Gem associated with Ireland" , "La couleur" , "and 1 Across: Ireland" .)

I've seen this clue in The Globe and Mail.

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