Crossword Genius

Is stony yet may be worth millions! (3)

Ross

I believe the answer is:

gem

I'm a little stuck... Click here to teach me more about this clue!

'be worth millions' is the definition.
The definition and answer can be both substances as well as being singular nouns.
Perhaps they are linked in a way I don't understand?

I don't understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for gem that I've seen before include "36 Across or any other jewel" , "Setter might bed this" , "A jewel of an Orangeman" , "Valuable stone" , "Pearl" .)

I've seen this clue in the Evening Standard.
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