Hard silica mineral (6)
I believe the answer is:
quartz
'hard silica mineral' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for quartz that I've seen before include "Stone such as amethyst or cairngorm" , "Hard mineral, crystal in watch" , "Mineral used in modern timepieces" , "Type of watch and crystal" , "Silicon dioxide mineral" .)