This criterion analyzes the surface of every crystal and how it interacts with light and reflection. There are many descriptive terms for luster: glassy, resinous, reflective, metallic, wet, shiny, et cetera. Radiance or reflected light, is highly appealing to the eye, like the gleam of gold, a glint of light, or the shimmer of water. While these are all very good for descriptions, we are more interested if a luster catches our eye, our attention, and makes a specimen more appealing. Dull luster is always a negative on main crystals, but in some instances, like on a matrix, it can enhance contrast.