The IOR (Index of Refraction) is quite simply, the amount of slowdown in the speed of light
in any medium. In a vacuum, the number is 1.0000. In any other material, the speed is slower,
and this difference causes any ray of light crossing the interface of materials to become bent.
IOR is a comprehensive database of over 300 IOR values, in an easily searchable format,
with examples and links to Wikipedia articles about those materials.
Take the kinks out of your raytracing, or add some in!