Lustre (mineralogy)

Lustre or luster 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

Adamantine lustre

Adamantine minerals possess a superlative lustre, which is most notably seen in diamond.[1] Such minerals are transparent or translucent, and have a high refractive index (of 1.9 or more).[2] Minerals with a true adamantine lustre are uncommon, with examples being cerussite and cubic zirconia.[2]

Minerals with a lesser (but still relatively high) degree of lustre are referred to as subadamantine, with some examples being garnet and corundum.[1]

Dull lustre

Dull (or earthy) minerals exhibit little to no lustre, due to coarse granulations which scatter light in all directions, approximating a Lambertian reflector. An example is kaolinite.[3] A distinction is sometimes drawn between dull minerals and earthy minerals,[4] with the latter being coarser, and having even less lustre.

Greasy lustre

Moss opal

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.[2] Many minerals with a greasy lustre also feel greasy to the touch.[5]

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,[6] pyrite[7] and magnetite.[8]

Pearly lustre

Pearly minerals consist of thin transparent co-planar sheets. Light reflecting from these layers give them a lustre reminiscent of pearls.[9] Such minerals possess perfect cleavage, with examples including muscovite and stilbite.[2]

Resinous lustre

Resinous minerals have the appearance of resin, chewing gum or (smooth-surfaced) plastic. A principal example is amber, which is a form of fossilized resin.[10]

Silky lustre

Satin spar variety of gypsum

Silky minerals have a parallel arrangement of extremely fine fibres,[2] giving them a lustre reminiscent of silk. Examples include asbestos, ulexite and the satin spar variety of gypsum. A fibrous lustre is similar, but has a coarser texture.

Submetallic lustre

Submetallic minerals have similar lustre to metal, but are duller and less reflective. A submetallic lustre often occurs in near-opaque minerals with very high refractive indices,[2] such as sphalerite, cinnabar, anthracite, and cuprite.

Vitreous lustre

Vitreous minerals have the lustre of glass. (The term is derived from the Latin for glass, vitrum.) This type of lustre is one of the most commonly seen,[9] and occurs in transparent or translucent minerals with relatively low refractive indices.[2] Common examples include calcite, quartz, topaz, beryl, tourmaline and fluorite, among others.

Waxy lustre

Waxy minerals have a lustre resembling wax. Examples include jade[11] and chalcedony.[12]

Other Languages
asturianu: Llustre
català: Lluïssor
español: Lustre
euskara: Distira
فارسی: جلا
français: Éclat
ಕನ್ನಡ: ಹೊಳಪು
Kreyòl ayisyen: Ekla
日本語: 光沢
norsk nynorsk: Glans i mineralogi
polski: Połysk
Simple English: Luster (mineralogy)
slovenčina: Lesk minerálu
slovenščina: Sijaj (mineralogija)
тоҷикӣ: Ҷило