A jewel in one sign of the zodiac (6)

I believe the answer is:

gemini

'sign of the zodiac' is the definition.
(Gemini is an example)

'a jewel in one' is the wordplay.
'a jewel' becomes 'gem' ('gem' can be a synonym of 'jewel').
'one' becomes 'i' (Roman numeral).
'gem'+'in'+'i'='GEMINI'

(Other definitions for gemini that I've seen before include "lot of stars" , "Zodiac sign - the Twins" , "Zodiacal twins" , "House" , "Castor and Pollux constellation" .)

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

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