The lustre of a metal is due to the presence of mobile (free) electrons in it.
The free electrons can move freely in the metal, causing any light incident to reflect. This reflection is a specular reflection rather than diffused, and thus the metal surface appears shiny or lustrous.
- Minerals produce metallic lustre with a refractive index of greater than 3. The minerals are opaque and usually are from the native element and sulphide groups.
- Adamantine lustre is the brilliant lustre produced by minerals such as diamond—the refractive indexes for this lustre range from 1.9 to 2.6.
- Vitreous lustre is the lustre of glass. The refractive indexes range from 1.3 to 1.9.
- Diamonds have a greasy lustre which is caused by the slight roughness of the surface.