Because it is made from natural resources, it does contain impurities. These impurities can cause broadband absorption if the quartz should solarize. Solarization appears as a purplish film distributed over the entire lamp envelope. The ionic impurities consist mostly of alkali earths, aluminum, and iron. The UV cut-off is 210 nm which makes this quartz suitable as a UV lamp material.
Solarization is part of the natural aging process.
This is a synthetic quartz grown from SiCl4. It is volatilized in an oxyhydrogen flame or, if a hydroxyl-free quartz is needed, it is formed by electrical arc in an oxidizing atmosphere. The type which is not hydroxyl free does have water absorption bands in the infrared. This type of quartz has the highest cost of all and the hydroxyl-free type costs are even higher. It does have the lowest UV cut-off at 160 nm.