Minerals may be categorized according to whether they are opaque or transparent. A thin section of an opaque mineral such as a metal will not transmit light, whereas a thin section of a transparent mineral will.
The absorption index of an opaque mineral is high. Light which is incident upon an opaque mineral such as a metal is unable to propagate through the mineral due to this high rate of absorption, and will thus be reflected. Opaque minerals typically reflect between 20% to 50% or more of the light incident upon them. In contrast, most of the light which is incident upon a transparent mineral passes into and through the mineral; transparent minerals may reflect as little as 5% of the incident light and as much as 20%. Typically those minerals which possess metallic bonding are opaque whereas those where ionic bonding is prevalent are transparent.
Relative differences in opacity and transparency are described as luster. The term luster refers to the quantity and quality of the light which is reflected from a mineral's exterior surfaces. Luster provides an assessment of how much the mineral surface 'sparkles'. This quality is determined by the type of atomic bonds present within the substance. It is related to the indices of absorption and refraction of the material and the amount of dispersion from the crystal lattice, as well as the texture of the exposed mineral surface.
Minerals are primarily divided into the two categories of metallic and nonmetallic luster. Minerals possessing metallic luster are opaque and very reflective, possessing a high absorptive index. This type of luster indicates the presence of metallic bonding within the crystal lattice of the material. Examples of minerals which exhibit metallic luster are native copper, gold, and silver, galena, pyrite, and chalcopyrite. The luster of a mineral which does not quite possess a metallic luster is termed submetallic; hematite provides an example of submetallic luster.
The property of streak can aid in distinguishing whether a specimen has a metallic or a nonmetallic luster. Metals tend to be soft, implying that more powdered material may be obtained from the streak sample of a metal than a nonmetal. Metals are also opaque, transmitting no light. Minerals which possess a metallic luster therefore tend to exhibit a thick, dense, dark streak whereas those which possess a nonmetallic luster tend to produce a thinner, less dense streak which is also lighter in color.
Adjectives such as "vitreous', 'dull', 'pearly', 'greasy', 'silky' or 'adamantine' are frequently used to describe various types of nonmetallic luster.
- Dull or Earthy
Minerals of dull or earthy luster reflect light very poorly and do not shine. This type of luster is often seen in minerals which are composed of an aggregate of tiny grains.
A surface of resinous luster possesses a sheen resembling that of resin. Such materials have a refractive index greater than 2.0. Sphalerite (ZnS) demonstrates a resinous luster.
Pearly luster appears iridescent, opalescent, or pearly. This is typically exhibited by mineral surfaces which are parallel to planes of perfect cleavage. Layer silicates such as talc often demonstrate a pearly luster on cleavage surfaces.
A surface which possesses greasy luster appears to be covered with a thin layer of oil. A light-scattering surface which is slightly rough, such as that of nepheline, may exhibit greasy luster.
Silky luster occurs when light is reflected off of an aggregate of fine parallel fibers; malachite and serpentine may both exhibit silky luster.
Vitreous luster occurs in minerals with predominant ionic bonding and resembles the reflective quality of broken glass. The refractive index of such minerals is 1.5 to 2.0. Many silicates possess this type of luster; quartz and tourmeline both demonstrate vitreous luster.
- Adamantine or brilliant
A brilliant luster such as the sparkling reflection of diamond is known as adamantine. Minerals of adamantine luster have high refractive indices (1.9-2.6) and are highly dispersive and translucent. Covalent bonding or the presence of heavy metal atoms or transition elements may result in adamantine luster.