Smoky Quartz Cluster Small

Quartz is the hard crystalline mineral composed of silica (SiO2). It primarily forms from molten magma, but can also be found in magmatic pegmatites, and as gangue in hot hydrothermal deposits. 
The color of smoky quartz is produced when natural radiation, emitted from the surrounding rock, activates color centers around aluminum impurities within the crystalline quartz. Amethyst forms in a similar way, from the activation of color centers around iron impurities.