As Gordon said, the rock is around 60% SiO₂, but this this is not in the form of quartz (the mineral with the formula SiO₂), but rather bound in silicates, such as plagioclase (CaAl₂Si₂O₈) or clinopyroxene (CaMgSi₂O₆). Separating the chemical SiO₂ from these minerals to produce commercial grade SiO₂ is very hard, expensive, and pointless. It is much easier to get SiO₂ from sandstones, beach sand, and quartzites, most of them are almost pure SiO₂ in the form of quartz. This is the case because the chemical grade SiO₂ we use in our own lab has small but higher than expected amounts of ZrO₂ in it. This basically means that it has a tiny bit of zircon (ZrSiO₄) in it. This is a mineral that's extremely resistant to chemical attack, and it accumulates in sedimentary detrital rocks, for example sandstone. Gives you an idea on where the SiO₂ used to make it came from...