Circular letter to a friend reputedly brings bad luck (4)

I believe the answer is:

opal

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'reputedly brings bad luck' is the definition.
I can't tell whether this defines the answer.

'circular letter to a friend' is the wordplay.
'circular letter' becomes 'o'.
'to a friend' becomes 'pal' (pal can mean a friend).
'o'+'pal'='OPAL'

Can you help me to learn more?

(Other definitions for opal that I've seen before include "Milky-white mineral" , "One in a ring perhaps" , "often semi-transparent" , "National gemstone of Australia" , "Translucent gemstone" .)

I've seen this clue in The Irish News.

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