Quartz crystals can be used as gems what other more practical uses can quartz crystals have?

Chemically it is Silica - Silicon Dioxide - the main constituent of glass.

Quartz is piezo-electric and was used in early sonar transducers, but modern sonar uses synthetic ceramics that are far more sensitive and efficient. (NB: don't be taken in by the purveyors of quartz "healing crystals" who seem to have latched onto this property: their claims for itare pure fraud.)

Decoratively, silica is the base part of semi-preciousmineral compounds (metal-silicates).