Rainbows and other atmospheric optics phenomena — very nice site, photographs and explanations.

Link

Glass Bead Bows:

This almost complete bow was produced by glass beads remaining on a dry road surface after resurfacing and painting. The bow is only about 21° in radius, half that of a rainwater bow because glass is more strongly refractive. The inside of the glass bead bow is brighter than the surroundings as is the rainbow. Glass bead bows can sometimes be seen on traffic signs - but take care if driving.

[Link]

Via Fark.