Though precious to some it's nothing to a friend (4)

I believe the answer is:

opal

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'though precious to some' is the definition.
I can't judge whether this defines the answer.

'nothing to a friend' is the wordplay.
'nothing' becomes 'O' (O resembles 0).
'to a friend' becomes 'pal' (pal can mean a friend).
'o'+'pal'='OPAL'

'it's' acts as a link.

Can you help me to learn more?

(Other definitions for opal that I've seen before include "Gem stone" , "Quartz-like gemstone" , "little jewel" , "Semiprecious stone of white and bluish colour" , "Type of gem" .)

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

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