Quartz

Fused quartz looks like glass, and has very useful properties. Unlike glass it is transparent in the near UV: a person could get a sun-tan behind quartz windows. The thermal coefficient of expansion is so nearly zero that white hot molten quartz can be sprayed into water to make tiny beads.

The most common use of quartz is to make envelopes for quartz-halogen bulbs.

Low pressure chlorine gas in the bulb returns evaporating tungsten to the filament, and the bulb can be operated at a very high temperature. Quartz halogen bulbs have some UV emission because of the very high black body temperature of the filament, and the transparency of the envelope.