Silica mineral (6)
I believe the answer is:
quartz
'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 "Hard mineral in timepieces" , "Hard glossy mineral present in many rocks" , "Colourless mineral used to make glass" , "Type of watch and crystal" , "Glassy mineral" .)