QUARTZ CRYSTALS vs. GLASS : what IS the difference? (www.reference.com)
****** Pure QUARTZ CRYSTALS are 100% silicon and oxygen (silicon dioxide), arranged in a crystalline structure. QUARTZ is permeable to visible light, but polarizes it (useful in lasers). QUARTZ is also piezoelectric (generates an electrical charge through mechanical stress), a quality used to vibrate QUARTZ, valuable in keeping precise time in watches and computer clocks.
****** Most GLASS, on the other hand, contains only 75% silicon dioxide plus additives NOT found in Quartz (such as lime or lead), is NOT crystalline in structure, is NOT piezoelectric. Visible light is NOT affected as it passes through GLASS.