It is usually found in top alluvia (4)

I believe the answer is:

opal

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'it is usually' is the definition.
The definition suggests a singular noun which matches the answer.

'in top alluvia' is the wordplay.
'in' says the answer is hidden in the clue.
'OPAL' is hidden in the letters of 'top alluvia'.

'found' is the link.

Can you help me to learn more?

(Other definitions for opal that I've seen before include "Birthstone for October" , "Opaque gem" , "Precious stuff" , "little jewel" , "Lustrous stone" .)

I've seen this clue in The Globe and Mail.

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