The term luster is used to describe the way a mineral surface looks when light is reflecting off of it. Luster is considered a basic descriptive parameter for minerals but varies somewhat even within a single crystal, which limits the term’s usefulness in categorizing gemstones and minerals. Luster in the mineral world is divided into two main types: nonmetallic and metallic. An intermediate type known as submetallic is also sometimes used to describe mineral luster. For the most part, the nonmetallic type of luster is used for gemstones because most do not meet the criteria for metallic or submetallic.
Examples of nonmetallic lusters commonly seen in the gemstone industry include vitreous, pearly, resinous, greasy, adamantine, silky, and dull. Vitreous, sometimes referred to as glassy, refers to a shiny, glass-like luster and is characteristic of most gem minerals, such as quartz and tourmaline. A pearly luster has an iridescent, pearl-like luster. Talc is an example of a mineral with a pearly luster. The term resinous is used to denote minerals that resemble resin, such as sphalerite. Minerals with a greasy luster appear as though they have a layer of oil or fat coating them. An example of a mineral with a greasy luster is graphite. An adamantine luster refers to a brilliant, mirror-like luster, such as the luster seen in diamonds. If a gem has a silky luster, this means that the fibers of the mineral are arranged in parallel, similar to the look and texture of silk. The final luster category, dull, is used for minerals that have little or no luster and scatter light in all directions, such as kaolinite.
Luster is a phenomenon of reflected light and is mostly due to the state of aggregation of the mineral. This means that one specimen could exhibit several different lusters, depending on what face is being examined. For example, gypsum may have a vitreous luster on some crystal faces, a pearly luster on surfaces parallel to the cleavage, and, if the mineral occurs in aggregates of long fiber, a silky luster. Because this mineral specimen displays a variety of lusters, luster can hardly be a useful diagnostic property in identifying gypsum!