Oddly curvy stalactite's 40% quartz (7)

I believe the answer is:

crystal

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'quartz' is the definition.
(quartz is a type of crystal)

I don't understand the remainder of the clue.

(Other definitions for crystal that I've seen before include "Clear material such as quartz" , "Type of glass often cut decoratively" , "Glass made at Waterford" , "Glassware" , "The fortune teller may use a . . . . . . . ball" .)

I've seen this clue in The Guardian.

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