Silica (SiO2, silicone dioxide) is the most common moulder of the vitreous lattice and is therefore the most important primary material for glass production. About half of the earth's crust is made up of silica (silicates and quartz), the main component in rock and sand. However, natural silica does not, generally, possess the necessary characteristics for glass production, because it forms complex minerals with other oxides (like, for example, in clays and feldspars with alumina, Al2O3), and because it contains some elements like iron which, even in small quantities, gives glass an undesired colour. Only silica that contains less than 0.1% of iron oxide (Fe2O3) can be used for sheet production. To produce table-glass or artistic glass, however, this percentage drops to 0.01% and only small quartz deposits guarantee this amount. For glass used in eyewear the acceptable level is even lower, less than 0.001%. It is a very small amount, the equivalent of 10 milligrams per kilogram of sand! The content of other minerals is even lower, like chrome, cobalt and copper etc. oxides which have the power to colour which is higher than the iron one. No natural sand can meet the needs of eyewear glass; for this reason, even sand from the best deposits must be purified further with special treatments.