Radiation-damage halos are present in quartz that surrounds, or is adjacent to, accessory minerals such as zircon. These halos are not visible in transmitted light but are revealed by cathodoluminescence (CL). They extend up to 45 μm away from the source of radioactivity. Some halos show concentric rings similar to those previously reported in pleochroic halos in biotite. Radiation-damage halos occur in three main settings: surrounding inclusions within quartz grains; penetrating into quartz from adjacent radionuclide-bearing grains; and in quartz cement near such grains. Halos are more common in pre-Paleozoic rocks than in younger rocks, except in cases of unusual enrichment in radioactive elements. Halos result from damage to quartz-crystal structure, identical to the process forming pleochroic halos in biotite. Damage is caused by alpha particles emitted by radioactive elements within mineral grains and is a partial function of time. Consequently, radiation-damage halos may have potential uses in geochronology.