20 ppm means that the silica has to be 99.998% pure. It is extremely hard to find silica that pure in nature.

More specifically:

Specific requirements as to tolerable limiting values differ from industry to industry. In the lamp tubing and optics industries aluminium content in the refined quartz concentrate should not exceed 20 ppm, other metals should be less than 1 ppm, and total impurities less than 30 ppm. For semiconductor base materials and crucibles aluminium content should be even lower, specified to less than 10 ppm, other metals less than 0.1 ppm, and total impurities not to exceed 15 ppm.

Feedstock for solar silicon used in the photovoltaic industry should generally have boron and phosphorus contents in the sub-ppm range since these elements are most difficult to remove and negatively affect the performance of the solar cells (Geerligs et al. 2002).

For microelectronics applications, e.g. in epoxy moulding compounds, uranium and thorium, both responsible for soft errors by alpha radiation, should be less than 2 ppb, and in low alpha applications, even less than 0.5 ppb.