Some are calm and inexpressive like a stone (9)

I believe the answer is:

almandine

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

'a stone' is the definition.
Both the definition and answer are singular nouns.
Maybe they are linked in a way I don't understand?

'some are calm and inexpressive' is the wordplay.
'some' indicates a hidden word (some of the parts of the word are used).
'ALMANDINE' is hidden within 'calm and inexpressive'.

'like' acts as a link.

Can you help me to learn more?

(Another definition for almandine that I've seen is " jewellery".)

I've seen this clue in The Hindu.

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