A gem, but friendless? (4)

I believe the answer is:

opal

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

'a gem' is the definition.
(I know that opal is a type of opaque gem)

I don't understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for opal that I've seen before include "Gem with delicate colours" , "it may be highly valued" , "Semi-precious stone of b;uish white colour" , "Delicately-coloured gem" , "Many-coloured gem" .)

I've seen this clue in The Times.

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