Lustre (Commonwealth English) or luster (American English; see spelling differences) is the way light interacts with the surface of a crystal, rock, or mineral. The word traces its origins back to the Latin lux, meaning "light", and generally implies radiance, gloss, or brilliance.
A range of terms are used to describe lustre, such as earthy, metallic, greasy, and silky. Similarly, the term vitreous (derived from the Latin for glass, vitrum) refers to a glassy lustre. A list of these terms is given below.
Lustre varies over a wide continuum, and so there are no rigid boundaries between the different types of lustre. (For this reason, different sources can often describe the same mineral differently. This ambiguity is further complicated by lustre's ability to vary widely within a particular mineral species). The terms are frequently combined to describe intermediate types of lustre (for example, a "vitreous greasy" lustre).
Some minerals exhibit unusual optical phenomena, such as asterism (the display of a star-shaped luminous area) or chatoyancy (the display of luminous bands, which appear to move as the specimen is rotated). A list of such phenomena is given below.
Common terms
thumb|upright|[[Diamond cut|Cut diamonds]]
Adamantine lustre
Adamantine minerals possess a superlative lustre, which is most notably seen in diamond. Such minerals are transparent or translucent, and have a high refractive index (of 1.9 or more). Minerals with a true adamantine lustre are uncommon, with examples including cerussite, zircon, and cubic zirconia. A distinction is sometimes drawn between dull minerals and earthy minerals, with the latter being coarser, and having even less lustre.
thumb|upright|Moss [[opal]]
Greasy lustre
Greasy minerals resemble fat or grease. A greasy lustre often occurs in minerals containing a great abundance of microscopic inclusions, with examples including opal and cordierite, jadeite.
thumb|upright|[[Pyrite]]
Metallic lustre
Metallic (or splendent) minerals have the lustre of polished metal, and with ideal surfaces will work as a reflective surface. Examples include galena, pyrite and magnetite.
thumb|upright|[[Muscovite]]
Pearly lustre
Pearly minerals consist of thin transparent co-planar sheets. Light reflecting from these layers gives them a lustre reminiscent of pearls.
thumb|upright|[[Satin spar variety of gypsum]]
Silky lustre
Silky minerals have a parallel arrangement of extremely fine fibres, and occurs in transparent or translucent minerals with relatively low refractive indices. and chalcedony.
Optical phenomena
thumb|upright=0.5|Sapphire
Asterism
Asterism is the display of a star-shaped luminous area. It is seen in some sapphires and rubies, where it is caused by impurities of rutile. It can also occur in garnet, diopside and spinel.
thumb|upright|[[Aventurine]]
Aventurescence
Aventurescence (or aventurization) is a reflectance effect like that of glitter. It arises from minute, preferentially oriented mineral platelets within the material. These platelets are so numerous that they also influence the material's body colour. In aventurine quartz, chrome-bearing fuchsite makes for a green stone and various iron oxides make for a red stone.
thumb|upright|[[Labradorite]]
Schiller
Schiller (German, literally "shimmer"), is the metallic iridescence originating from below the surface of a stone that occurs when light is reflected between layers of minerals. It is seen in moonstone and labradorite and is very similar to adularescence and aventurescence.
References
ja:光沢
pl:Połysk
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