Crystalline mineral (6)
I believe the answer is:
quartz
'crystalline mineral' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for quartz that I've seen before include "Type of watch and crystal" , "Hard glossy mineral present in many rocks" , "Stone such as amethyst or cairngorm" , "Hard silica mineral" , "Colourless mineral used to make glass" .)