Transparent mineral (7)

I believe the answer is:

crystal

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

This is all the clue.

(Other definitions for crystal that I've seen before include "The fortune teller may use a . . . . . . . ball" , "Type of clear-cut, lucent glass" , "Glass made at Waterford" , "Regular-shaped mineral piece" , "Highly transparent glass" .)

I've seen this clue in the USA Today, the Sydney Morning Herald, The Guardian, The Sun and The Times.

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