The way in which a mineral reflects light is known as the mineral's luster. Luster is a key aspect of the identification of different minerals and provides geologist with a visual classification system.
There are four major types of luster used in mineral identification. Minerals said to have a metallic luster have high reflectance and are opaque, which means light does not shine through them. Minerals with a glassy luster also have high reflectance but are translucent or transparent, which means light passes through the minerals. Pearly minerals have low reflectance and are either translucent or opaque. Dull minerals have no reflectance and are also opaque. A mineral may have a luster that shows aspects of more than one type.