Mineral substance, optimal but nameless (4)

I believe the answer is:

opal

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

'mineral' is the definition.
(I've seen this before)

'substance optimal but nameless' is the wordplay.
I cannot quite see how this works, but
'substance' could be 'pa' (Pa is an example) and 'pa' is found within the answer.
This explanation may well be incorrect...

Can you help me to learn more?

(Other definitions for opal that I've seen before include "Milky white gem" , "Variegated gemstone" , "Shimmering stone" , "it may be highly valued" , "Mineral, usually iridescent" .)

I've seen this clue in the Evening Standard.

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