A gem of a language (4)

I believe the answer is:

ruby

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'a gem' is the definition.
(ruby is a kind of gem)

I cannot understand how the remainder of the clue works.

(Other definitions for ruby that I've seen before include "killed an assassin" , "A red jewel" , "Jack perhaps" , "Stony girl" , "Completely" .)

I've seen this clue in The Hindu.

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