Morphologically and photometrically anomalous patches of highly-reflective material exist inside several large mercurian craters. Calculations assuming that the patches exhibit an average lunar photometric function yield normal albedos of .39--.45, roughly 60% higher than Aristarchus, the brightest feature on the Moon. Color-ratio images derived from Mariner 10 imaging data indicate that bright patches are bluer than typical mercurian surface material, and are surrounded by material redder than typical surface material. Local physico-chemical alteration along impact-induced fractures may have been involved in production of these uniquely mercurian features.