Scientifically speaking, luster refers to how light reacts with the surface of a rock, crystal, or mineral. It comes from the latin word “lux” which means light. It generally implies gloss, radiance, and brilliance. There are a wide range of expressions that have similar meaning with luster. These include metallic, earthy, silky, and greasy. It could also mean the thin coating which contains unoxidized metal which activates an iridescent glaze among types of ceramics.