Luster is a physical property of metals. It refers to the way a metal's surface reflects light. Almost all metals are lustrous, while elements such as gases are not.
Luster is a property that can help identify an unknown mineral by observing one of its freshly broken surfaces. For metallic minerals, some of the terms that can be used to describe luster are "shiny," which means the metal's surface is very reflective, and "dull," which means there is no reflection from the surface at all. For nonmetallic minerals, some examples of descriptive words used are "vitreous," which means the gem's surface is glasslike, and "adamantine," which means the mineral's surface is brilliant.