High explosive driving systems and high‐speed optical techniques have been used to investigate the compressional behavior ofx‐cut,y‐cut, andz‐cut quartz crystals and fused quartz up to about 750 kbar pressure. In both forms of silica two‐wave structures over a wide pressure range are identified as resulting from unusually high Hugoniot elastic limits. In crystalline quartz a dependence of the pressure of these elastic limits on driving shock pressure and sample thickness is associated with a relaxation‐time phenomenon connected with the transition from elastic to fluid‐like flow. Recovery experiments are reported which confirm a previous observation that sufficiently high shock pressures result in the transition of crystalline quartz to the fused state, and the implications of the compression data in this regard are discussed. Observations on fused quartz deomonstrate that shock‐wave loading results in both the anomalous compression at low pressures and the compaction phenomena above 100 kbar which have been reported from hydrostatic experiments.