Luster is considered a basic descriptive parameter for minerals but varies somewhat even within a single crystal, and its usefulness is therefore limited. Lusters include: vitreous (the luster of glass— characteristic of most gem minerals); pearly (iridescent, pearl-like); resinous (luster of resin); greasy (appears covered by oil layer); adamantine (hard, steely brilliance like the reflection from a diamond); silky (fibrous reflection of silk); dull. Luster is a phenomenon of reflected light and is mostly due to the state of aggregation of the mineral. For example, gypsum may have a vitreous luster on some crystal faces; the luster is pearly on surfaces parallel to the excellent cleavage of this mineral; and if the mineral occurs in aggregates of long fibers (satin spar), it has a silky luster. Luster can hardly be a useful diagnostic property in identifying gypsum under these circumstances! Luster is primarily divided into two types: metallic and nonmetallic. There are also intermediate types, called submetallic. Any mineral that does not have a metallic appearance is described as nonmetallic, and the above descriptive terms are applied.