Oddly curvy stalactite's 40% quartz (7)
I believe the answer is:
crystal
'quartz' is the definition.
(quartz is a type of crystal)
I don't understand the remainder of the clue.
Can you help me to learn more?
(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" .)