Quartz Line filters are made of 99.9 percent pure fused-quartz glass for unrivalled durability and optical clarity. These single lens element filters produce a very low refractive index of just 1.46 and offer outstanding temperature stability. Combined with an over-engineered brass frame, Quartz Line filters thread on smoothly and are built to withstand your most rugged adventures.82 mm Quartz Line Circular Polarizer Filter99.9 percent pure fused-quartz glass for unrivalled durability and optical clarityQuartz Line filters are made of 999% pure fused-quartz glass for unrivalled durability and optical clarity