It’s found in rock — after support it gets to grow (5)

I believe the answer is:

agate

'it's found in rock' is the definition.
(agate is a type of stone)

I cannot understand how the remainder of the clue works.

(Other definitions for agate that I've seen before include "Banded ornamental stone" , "Type of quartz" , "Quartz variety" , "Onyx" , "Semi-precious quartz" .)

I've seen this clue in The Guardian.

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