The alkali-silica reaction in opal was studied in the SEM with backscattered electrons, combined with quantitative image analysis, and also in an electron microprobe. A system of cracks was observed in the bands of gel in the opal, ahead of narrow fingers of gel into the unreacted opal and running across the aggregate-cement paste interface into the cement paste. There appeared to be good adhesion of the cement paste to the opal at the interface. Chemical analysis of the gel indicated the presence of calcium-rich gel with various compositions (CaO:SiO2 = 0.5-1.0) in the opal. There was considerable variation in the concentrations of K2O and Na2O in the gel. The analyses of the gel fell into three distinct composition ranges, with distinct boundaries between gels of different composition. Close to the interface there was depletion of calcium in the cement paste inner product.