Most cinnabar is brilliant red and remains so under sunlight. However, some cinnabar is photosensitive and irreversibly blackens during exposure to sunlight. Electron microprobe analyses of cinnabar from several occurrences show that photosensitive cinnabar contains noteworthy concentrations of chlorine, whereas non-photosensitive cinnabar does not. Experimental evidence shows that non-photosensitive cinnabar becomes photosensitive after exposure to halogens. The darkening of cinnabar in sunlight is caused by the presence of contained chlorine or other halogens.