Grains which contain two or more crystals are known as polycrystalline quartz. Syntaxial secondary overgrowths picked out by 'dust pellicles' are common. May be monocrystalline or polycrystalline. Polycrystalline grains tend to be larger. Undulose extinction common.
Inclusions generally uncommon.
Derived from acid volcanic rocks. Typically show b -quartz habit of hexagonal bipyramids. Often euhedral, with rounded corners and resorption embayments. Derived from hydrothermal veins, pegmatites etc. Zoned inclusions and vermicular chlorite are diagnostic.