What causes the blue color in quartz?
The blue color in quartz results from Rayleigh scattering in the quartz ( similar to why the sky is blue). If you have a thin slab of the material, it will be blue in reflected light, while it will be orangeish-brown in transmitted light. In studies of blue quartz from the Llano uplift in Texas, inclusions of rounded ilmenite with a diameter of 0.06 mm at a concentration of 125 per cubic micrometer were found by TEM. A similar type of quartz was also present in the Roseland district of Virginia. ( see Zolensky et al 1988 American Mineralogist pp313-323) There have been some synthetic blue quartz colored by cobalt or iron, but these are not known to occur naturally. Gordon Nord wrote: Rayleigh scattering requires a refractive difference between the host mineral (quartz) and the inclusion (ilmenite). Also the inclusions must be 1/4 the wavelength of light to backscatter blue visible wavelengths. This means the ilmenite precipitates in quartz must be on the order of 100 nm or 1/4 of 4000
