an optical effect caused by the reflection of part of the light flux incident to the mineral. Mineral luster depends on the average refractive index of the mineral (N), to which the reflectance index (R) is related by the Fresnel equation:
Minerals with metallic, metalliform, and nonmetallic luster are distinguished. Metallic luster is characteristic of nontransparent, native metals (gold, silver, copper, and so on), many sulfur compounds (for example, galenite and chalcopyrite), and metal oxides (magnetite, pyrolusite, and others). Metalliform luster resembles the luster of tarnished metal surfaces. Nonmetallic luster is characteristic of transparent minerals. The following types of it are distinguished: adamantine (diamond, cinnabar, and so on), glasslike (quartz, gypsum, and calcite), oily (native sulfur), pearly (mica and gypsum), and silklike (asbestos). In certain minerals the luster on the crystal faces and on a fracture are different. For example, for quartz the luster is glasslike on the faces and oily on the fracture. Thin films on an old surface and incrustations of foreign substances also sharply alter mineral luster.