Quartz crystal filters are made of silicon dioxide and exhibit piezoelectric characterisitcs. The advantage of using quartz, as opposed to some other hard material, is the inherent piezoelectric effect, which enables mechanical oscillations to be directly converted into oscillating electrical signals. Quartz crystals also exhibit minimal thermal expansion, which ensures that the frequency remains constant across a wide temperature range. Ultimately, quartz crystal solutions offer significantly higher quality performance than LCR filters!