One form of carbon, slangily, is unstable (5)

I believe the answer is:

rocks

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'is unstable' is the definition.
Although both the answer and definition are verbs in their -s form, I don't see how they can define each other.

I can't explain the remainder of the clue.

Can you help me to learn more?

(Other definitions for rocks that I've seen before include "Excites" , "Large stones" , "Boulders" , "Regular moves to get child off" , "Causes shock to; diamonds" .)

I've seen this clue in The Guardian.

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