Gem of an isle? (7)

I believe the answer is:

emerald

I believe this is a double definition.

'gem' is the first definition.
(emerald is a kind of gem)

'an isle?' is the second definition.
(Emerald Isle is an example)

'of' is the link.

(Other definitions for emerald that I've seen before include "Bright green colour" , "Mere lad (anag.)" , "Highly-valued variety of beryl" , "a type employed by the press" , "Shade of green" .)

I've seen this clue in The Telegraph and The Sun.

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