Luster is the way the surface of a stone reflects light. It depends mostly on the refractive index and the quality of the polish.
In more detail
Diamond has adamantine luster. Moissanite's is subadamantine. Most colored stones, such as emerald, ruby and spinel, have vitreous (glass-like) luster.
Related terms
- Refractive index: Refractive index is the speed of light in a vacuum divided by its speed in the stone. A higher index bends light more where it crosses a surface at an angle.
- Fresnel reflection: Fresnel reflection is the share of light that a surface reflects instead of letting through. The Fresnel equations give that share from the two refractive indices, the angle and the polarization.
- Polish: Polish is the grade for the smoothness of the facet surfaces, from Excellent to Poor. The grader looks at 10x for pits, nicks, polish lines, scratches, burn marks and rough patches.