Working granite when one’s missing more colourful stone (6)

I believe the answer is:

garnet

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'stone' is the definition.
(precious stone)

I don't understand the rest of the clue.

(Other definitions for garnet that I've seen before include "Deep red gem stone" , "Semi-precious stone of argent type" , "Glassy red, yellow or green gemstone" , "Hard glassy mineral used as a gemstone" , "Deep red gemstone" .)

I've seen this clue in The Times.

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