Chalcopyrite and quartz crystals. Close-up of a specimen of rock containing crystals of the minerals chalcopyrite and quartz. Chalcopyrite is a copper iron sulphide with a brassy to golden yellow colour, and is an important source of copper. Quartz is a form of silica (silicon dioxide) characterized by its hexagonal (six-fold) crystal symmetry. It is the second most abundant mineral in the Earth's crust after feldspar. It is colourless and transparent when pure but ranges from blue to pink in colour owing to a variety of natural chemical impurities.