The terms used by the Gemstone Industry can be technically sounding terms and confusing to a purchaser. Here is some information about the Refractive Index and Dispersion of gemstones.
Refractive Index of Gemstones.
When light passes from one density or type of material (air) into another (gemstone), it is bent or "refracted". The amount, or degree, that the beam of light is bent will be determined by the density difference between the air and the gem. The measurement used to quantify the amount that the light is bent is the "refractive index" (RI).
The refractive index of Labradorite gemstones is between 1.555 and 1.575. Lapis Lazuli gemstones and Obsidian gemstones have an RI of 1.500 where as Orange River Quartz gemstones have an RI of 1.544 to 1.553.
Dispersion of Gemstones – is the separation of white light into its component colours.