Stone one may think of in connection with strata (3)

I believe the answer is:

gem

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'stone' is the definition.
(a gemstone)

I don't understand the rest of the clue.

(Other definitions for gem that I've seen before include "Pleasant and cheerful (from German)" , "Kindness; comfort (German)" , "Emerald or sapphire, say" , "A jewel of an Orangeman" , "Precious or semi-precious stone" .)

I've seen this clue in the Evening Standard.

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