Lustre

Luster describes how the surface of a mineral reflects light. This property is completely independent of the mineral's color. In mineralogy, the most common lusters are vitreous, greasy, metallic, silky, adamantine, pearly, pitchy, resinous, waxy, and earthy.

The luster of a mineral depends on both its atomic and macroscopic structure. The most important number for quantifying luster is the refractive index (n) . This indicates how strongly light waves are reflected when they strike the mineral. Thus, a high refractive index (> 1.8) results in an adamantine luster, and a medium to low refractive index results in a vitreous luster (1.3 - 1.9).

Metallic bonds with their free electrons in metallic minerals (e.g., pyrite) cause an incoming light beam to penetrate only slightly into the mineral, with up to 99% of the light being reflected at the surface (metallic luster). Other minerals, such as (PbCO₃) also exhibit a strong luster (diamond-like luster). The reason for this lies in the lead atom, which contains a relatively large number of electrons in its outer shells. This makes the entire electron shell highly polarizable, meaning it is easily deformed by the electric field of the incoming light. This deformity absorbs energy from the light wave, strongly refracts it within the mineral, and slows it down. As a result of this high optical resistance, a large portion of the light is reflected directly at the surface. In cerussite, for example, light travels at only about 55% of its normal speed in a vacuum. Even minerals that contain no metals at all can exhibit an intense luster. The best example is diamond, in which carbon atoms are connected by short, covalent bonds. When light enters the diamond lattice, it encounters an immense density of electron pairs, causing it to be massively slowed down and reflected (diamond luster). Within the diamond, light travels at only about 41% of its normal speed in a vacuum.

Other types of luster arise from macroscopic structures. For example, a fibrous structure of the mineral causes light to be scattered perpendicular to the fiber direction, resulting in a silky luster. In minerals with perfect, platy cleavage, light is reflected at all thin layers at different depths, and the reflected light waves interfere with each other. Our eyes perceive this as a soft, white, pearly luster.