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Silica (SiO2) aerogel is widely used in high-energy-density shock experiments due to its low and adjustable density. Reported here are measurements of the shock velocity, optical radiance, and reflectivity of shocked SiO2 aerogel with initial densities of 0.1, 0.2, and 0.3 g/cm3. These results are compared with similar data from three solid polymorphs of SiO2, silica, quartz, and stishovite with initial densities 2.2, 2.65, and 4.3 g/cm3, respectively. Interestingly, below a brightness temperature of Tbright≈35,000 K, the slope of the radiance vs shock velocity is the same for each of the SiO2 aerogels and solid polymorphs. At Tbright≈35000 K, there is an abrupt change in the radiance vs shock velocity slope for aerogels, but not seen in the solid polymorphs over the pressures and temperatures explored here. An empirical model of shock front radiance as a function of SiO2 density and laser drive parameters is reported to aid in the design of experiments requiring maximum shock front radiance.