Cultured pearl, pure Natal (9)

I believe the answer is:

puerperal

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'natal' is the definition.
Both the definition and answer are adjectives. Perhaps there's an association between them I don't understand?

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Another definition for puerperal that I've seen is " after birth of child".)

I've seen this clue in The Times and The Sunday Times.

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