Lustre (mineralogy)

Lustre (or luster) is a description of the way light interacts with the surface of a crystal, rock, or mineral. The word lustre traces its origins back to the Latin word lux, meaning "light", and generally implies radiance, gloss, or brilliance.

A range of terms are used to describe lustre. Many terms refer to materials having similar lustres, 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 terms. (For this reason, different sources can often describe the same mineral differently. This ambiguity is further complicated by the fact that lustre can 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

Cut diamonds

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 zircon.^[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

Kaolinite

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

Metallic luster

Pyrite

Metallic (or splendent) minerals have the lustre of polished metal (and with ideal surfaces will act like a mirror). Examples include galena,^[6] pyrite^[7] and magnetite.^[8]

Pearly luster

Muscovite

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

Amber

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

Sphalerite

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 and cuprite.

Vitreous lustre

Quartz

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

Jade

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

Optical phenomena

Asterism

Sapphire cabochon

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.^[12][13] It can also occur in garnet, diopside and spinel.

Aventurescence

Aventurine

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.^[12]

Chatoyancy

Tiger's eye

Chatoyant minerals display luminous bands, which appear to move as the specimen is rotated. Such minerals are composed of parallel fibers (or contain fibrous voids or inclusions), which reflect light into a direction perpendicular to their orientation, thus forming narrow bands of light. The most famous examples are tiger's eye and cymophane, but the effect may also occur in other minerals such as aquamarine, moonstone and tourmaline.

Colour change

Alexandrite

Color change is most commonly found in Alexandrite, a variety of chrysoberyl gemstones. Other gems also occur in color-change varieties, including (but not limited to) sapphire, garnet, spinel. Alexandrite displays a color change dependent upon light, along with strong pleochroism. The gem results from small scale replacement of aluminium by chromium oxide, which is responsible for alexandrite's characteristic green to red color change. Alexandrite from the Ural Mountains in Russia is green by daylight and red by incandescent light. Other varieties of alexandrite may be yellowish or pink in daylight and a columbine or raspberry red by incandescent light. The optimum or "ideal" color change would be fine emerald green to fine purplish red, but this is exceedingly rare.

Schiller

Labradorite

Schiller, from German for "twinkle", is a term used to describe 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.^[14]

References

1. ^ ^{a b} GIA Gem Reference Guide. Gemological Institute of America. 1995. ISBN 0-87311-019-6. 
2. ^ ^{a b c d e f g} Duda, Rudolf & Rejl, Lubos (1990). Minerals of the World. Arch Cape Press. ISBN 0-517-68030-0. 
3. ^ "Webmineral: Kaolinite Mineral Data". http://www.webmineral.com/data/Kaolinite.shtml. Retrieved 2008-06-21. 
4. ^ Hankin, Rosie (1998). Rocks, Crystals & Minerals. Quintet Publishing. ISBN 1-86155-480-X. 
5. ^ "Emporia State University: GO 340 Gemstones & Gemology: Visual Properties". http://www.emporia.edu/earthsci/amber/go340/visual.htm. Retrieved 2008-06-19. 
6. ^ "Webmineral: Galena Mineral Data". http://webmineral.com/data/Galena.shtml. Retrieved 2008-07-05. 
7. ^ "Webmineral: Pyrite Mineral Data". http://webmineral.com/data/Pyrite.shtml. Retrieved 2008-07-05. 
8. ^ "Webmineral: Magnetite Mineral Data". http://webmineral.com/data/Magnetite.shtml. Retrieved 2008-07-05. 
9. ^ ^{a b} "Optical properties of Rocks and Minerals". http://www.rockcollector.co.uk/opticalprop.htm. Retrieved 2008-06-01. 
10. ^ "Webmineral: Amber Mineral Data". http://webmineral.com/data/Amber.shtml. Retrieved 2008-06-21. 
11. ^ "Emporia State University: GO 340 Gemstones & Gemology: Jade". http://www.emporia.edu/earthsci/amber/go340/jade.htm. Retrieved 2008-07-14. 
12. ^ ^{a b c} Bonewitz, Ronald Louis (2005). Rock and Gem. Dorling Kindersley. pp.  152 – 153. ISBN 0-7513-4400-1. 
13. ^ Emsley, John (2001). Nature's Building Blocks: An A-Z Guide to the Elements. Oxford University Press. pp.  451 – 53. ISBN 0-19-850341-5. 
14. ^ Shipley, Robert M. (2007). Dictionary of gems and gemology. Read Books. p. 93. ISBN 0873110072.